Forgive my long answer, but your question covers a lot of territory.
First off, all three are good cameras.
Secondly, do NOT get a 50D right now. The 60D is due to be announced soon, so you could either look at that or get a 50D at reduced price once it's announced.
I would take a good look at how much you want to spend. Getting the nicest (7D, without a doubt) body is tempting, but it's much smarter to start off getting high quality lenses. In most situations you're not going to get better image quality with a 7D over a T2i, but with a $1000 lens vs a $500 lens, you will see a large quality difference.
As far as what lenses to get, that really depends on what you want to shoot. A few suggestions.
No matter what you want to do, I'd suggest getting the Canon 50mm f/1.8 II. This lens runs around $120, and is the best quality lens you can buy. It is fantastic for a range of things, but is notably good for portraits and any low light situation.
If you do want to do portraits, the lens range (and when I say range, I'm talking focal length, or mm) you want to look for is around 50mm to 150mm. You do not want to use a wide angle for portraits, as it will cause distortion in a persons features that is more often than not un-attractive. The 50mm f/1.8 is a great lens for portraits, other good ones include the 85mm f/1.8 (if you want to be a bit further away) If you want to spend a but more money, there's the 50mm f/1.4, which is higher quality, focuses faster, and is slightly better in low light than the 50mm f/1.8. The very high end lenses (which, in the Canon lineup is denoted by an L in the name, and a red ring around the end of the lens) you're looking at the 50mm f/1.2L, 85mm f/1.2L, the 135mm f/2L, and to some extent the 24-104 f/4L and the 24-70 f/2.8L. All of these lens run at least $1000 and you should do a lot of research before settling on one.You'll notice that most of the lenses I recommended are prime, not zoom lenses. (primes being fixed focal length, such as 50mm, and zooms having a variable focal length, such as 24-70.) Portrait photographers use primes a lot more because they are sharper, and allow for better control of background blur.
If you like to do nature, wildlife, and landscapes, you'll want two lenses, something wide to capture a good view, and a powerful telephoto lens to bring you closer to wildlife. Two suggestions I have are the Canon 10-22mm the Canon 70-300. The 10-22mm is an EF-S lens, meaning it will only work on cameras that have APS-C sized sensors. Those being the Rebel cameras, 10/20/30/40/50D, and the 7D. If you ever get a 5D series or 1D series camera, you will not be able to put that lens on it. The 10-22 is well known for being, first, very wide, and second, very high quality. On the telephoto side, I'd go with at least a Canon 70-300, if not something more powerful. I personally use a Sigma 150-500 lens.
If you want to do macro work as well, at the least you'll want a set of extension tubes. These run about $130 (look for Kenko brand ones). these go in between a lens and the camera, and give the lens macro capabilities. They do cut out light in the process. If you want to do a lot of macro, you want a macro lens. The Canon 60mm macro is a good start, however it is EF-S like the 10-22 I mentioned above, thus it wont work on 5D and 1D series cameras. The canon 100mm f/2.8 is also a very good macro lens, and sigma makes several good macro lenses as well.
There are a LOT more lenses out there, trust me.
I would also recommend getting a flash, as it can really improve your photography. Make sure whatever you get has manual functions, tilt, and swivel, otherwise you're just getting a more powerful version of the pop-up on the camera. The Canon 430EX II is a great place to start, but there are other makers of flashes as well.
Also, there's video. If you don't care about your DSLR having video' I'd recommend waiting for the 60D to come out and then buying a discounted 50D. The 60D will have video.
Otherwise, I would actually suggest staying away from the 7D. It is a fantastic camera (it's one of the cameras I own and use on a regular basis) but most of its features are things you probably wouldn't be able to appreciate at this point. I think you'd be much happier in the long run getting a T2i, 50D, or 60D and spending more money on lenses. You will be able to notice the difference much more, and better yet, lenses don't get outdated nearly as fast, so they're very good investments.
www.dpreview.com is great resource for reviews, as well as a great side-by-side comparison of camera features (look under buying guide). There are tons of other great sites out there to help you choose.
Check out http://photography-on-the.net/forum/. Thats a forum dedicated specifically to Canon cameras, and the people there are extremely helpful.
Also, if you have any questions about what I said here, or further questions about this in general, you can send me an email on my profile.
I hope this helps!
6 years of photography, working in a photo store for 2 years