First and foremost, BioWare has updated their Mass Effect page to reflect the oncoming sequel. You can find an assortment of neato information on new things at [url="http://masseffect.bioware.com/"]the official site[/url], including pre-order information and profiles on new areas and characters that you will be experiencing throughout the game.
You can also pre-order at [url="http://www.amazon.com/Mass-Effect-2-Xbox-360/dp/B001TORSII"]Amazon.com[/url] and some GameStop locations (check with your local retailer to find out more). As of right now (6/6/09) there is no official release date aside from "early 2010". BioWare's Chris Priestly addresses the "release date" matter on some sites [url="http://meforums.bioware.com/viewdevposts.html?topic=680221&forum=144"]here[/url], saying they are simply estimates and no time frame has been officially announced yet.
What is going on in ME2?
Naturally BioWare doesn't want to spoil all the surprises, but they have been surprisingly generous with the current premise of the game. ME2 supposedly takes place two years after the siege on the Citadel. In that time a new threat has emerged and is attacking the civilized races of space. You, as Commander Shepard, are to undertake what is presumed to be a suicide mission. You'll be collecting the most hardcore characters of the galaxy to assist you, but even then the situation looks bleak.
What is happening with the squad members?
As of right now BW is being very tight-lipped about your team aside from the fact there will be 12 joinable squad members (see the official FAQ). We do know of at least three confirmed NPCs who will be potential hire-ons.
From the iPhone tie-in game we have two new squadmates, Jacob and Miranda. Jacob is a biotic soldier and survivor of Eden Prime. Miranda is still an unknown, but from E3 footage of her abilities we can deduct she is a sentinel-like class. She also dons a Cerberus logo on her shirt, indicating her affiliation with them. They are featured in the E3 demo [url="http://e3.g4tv.com/videos/38775/Mass-Effect-2-Preview-Hands-On/"]here[/url] as Shepard's squad.
One of the specialists you will encounter is Thane, [url="http://www.gamersyde.com/leech_11528_en.html"]a drell assassin[/url] (hit 'see the stream' - warning: spoiler content in the link). He looks suspiciously like Abraham Sapien if you ask me.
What about the original squad?
Presumably to appeal to new gamers that have not played ME1, BioWare hasn't shown much about the first game's content or characters. As such this has lead to a lot of worry about their presence in the sequel. However, there have been several promising signs.
In the June issue of OXM an ME2 exclusive talks about a "former squad member proficient in sniper rifles" making an appearance. Given there were only two characters capable of using sniper rifles, and the article used the prefix 'he', we can assume this is Garrus Vakarian.
Several official screenshots on the main site include a female quarian that looks like Tali (same pattern on her veil, coloration of clothing) but Christ Priestly points out that it's "highly unlikely Tali is the only female quarian in the flotilla". As such we don't know if it's Tali or not.
As for the fate of your romances, Mr. Priestly has recently stated that Liara, the romance of the "official" Shepard, [url="http://meforums.bioware.com/viewdevposts.html?topic=680711&forum=144"]makes an appearance[/url] in some [url="http://e3.gamespot.com/video/6211391/"]recent teaser footage[/url]. He goes on to add that Liara would be replaced by Ashley or Kaidan respectively in that scene dependent on your choices (supposing you romanced anyone). How much of a role the romance will play in the story is unknown.
Anything else of interest?
In addition to the numerous tweaks of combat there have been some official announcements on the official forums.
According to developer Brennan Holmes [url="http://meforums.bioware.com/viewdevposts.html?topic=680749&forum=144"]medi-gel is gone for good[/url]. He states this was done as a way to get the players to think more tactically in combat.
Now some bad news for players who have spent countless hours building up the commander in preparation for ME2: Christina Norman, a senior programmer, has confirmed that [url="http://meforums.bioware.com/viewdevposts.html?topic=680892&forum=144"]Shepard's level/build will not be carried over to the sequel[/url]. It has also been mentioned that even imported Shepards will encounter a new "character creation" screen and will reselect their class/appearance.
Concerning a "reload" animation some fans noticed in combat footage Preston Watamaniuk clarifies that this [url="http://meforums.bioware.com/viewdevposts.html?topic=680714&forum=144"]the discharging of a heat sink[/url], and certain classes gain benefits for managing hot weapons properly.
In a particularly insightful number of posts Brennan Holmes [url="http://meforums.bioware.com/viewdevposts.html?topic=680659&forum=144"]talks about the weapons system[/url] and how it has changed. Presumably characters will already be proficient in the weapons of their respective class and can tinker around with some non-proficient weapons. Now you will not be carting around the standard group of 4 guns but only will carry those you need. One demo shows a new party member with two pistols in her radial menu rather than the standard pistol/assault rifle/sniper rifle/shotgun line up.
Speaking of the weapons, we have gone from 4 categories of guns to a whopping 19. Miss Norman explains [url="http://meforums.bioware.com/viewdevposts.html?topic=680233&forum=144"]here[/url] that classes will still have access to their traditional weapons (an Infiltrator has access to pistols and sniper rifles) in addition to several new types of guns. The claim is that each gun "handles" differently and will fill unique to the player. These also include large guns like missile launcher and mini nukes (Fallout 3 anybody?), but the devs assure us that there are heavy restrictions on more powerful guns.
That's all for now. I'll tack on any further points of interest I come across as I see them.