The Canon 5D3 is out and is EPIC, but also $3500 and super far out of my price range. But, with the release of the 5D3, the 5D2 - still a truly amazing machine after 4-ish years on the market - is dropping like a brick in the price department.
Been doing some research to see if it actually makes sense for me to upgrade to a full-frame at some point. Given the dropping price of the 5D2 (going for ~$1600 on Craigslist or Ebay) it seems a good thing to think about.
I found articles on both sides of the issue. This one: http://www.extremeinstability.com/topic-full-frame.htm argues against full-frame, discussing the issues of vignetting on the frame edges and that the quality of the lens declines as you get closer to the edges. A cropped sensor does not use the edges of the frame, and so you don't have those issues.
This article from Ken Rockwell http://www.kenrockwell.com/tech/full-frame-advantage.htm takes the completely opposite stance, voting strongly for the quality of Full-Frame and suggesting much greater clarity/sharpness, colour differentiation, and low noise giving you the ability to get more out of a given ISO setting.
Finally, this article is less biased one way or another http://digital-photography-school.com/full-frame-sensor-vs-crop-sensor-which-is-right-for-you and seems to just encourage you to pick a camera based on how you plan to use it. It suggests a cropped sensor if you take a lot of wildlife photos or sports, since the cropped sensor in effect gives you a free multiplier on your zoom lenses. It suggests a full-frame if you take a lot of photos of architecture or landscapes and might benefit more from more use on the wider-angle spectrum.
Given a quick peruse of my favourite photos of the last few years (http://www.mikejutan.com/photos/) my broad style of photos seems to cover roughly: architecture, people, portraits, nature, landcapes. These mostly fall in the wide-angle camp and there are some close-ups, but they are all shot at pretty close range and not zoomed-in from afar (the effect that you would get from a cropped sensor).
So that seems somewhat conclusive. I'd be interested to make sure I don't end up with the vignetting issues that the first guy talks about, and perhaps that means dropping some further bills on a 28-105 f/4 lens which is certainly gonna increase the "upgrade cost". So it's a tough call, but I've been a big fan of the 5D2 for a long time and I'd be interested to trade-up from my T1i if the time was right.
I think the best advice comes from Ken Rockwell (and, my buddy Schmuel Zimmer): the camera (and even the lens) isn't gonna be what makes you good, and isn't gonna be what makes you a better photographer. Only practice will do that. Joel suggested upgrading "only if I think it's gonna make me go out and shoot more" - that's a great suggestion. I don't know if that's definitely the case or not, likely not. But, on the other hand, I'm starting to plan for more photo hikes with my buddy Justin and reading more about night photography, and just bought a pretty cool ball-head swivel tripod. So it does seem like I'm enjoying this more and more as time goes on. A photo class might be a good idea, and I also want to spend a solid amount of time learning better post-processing skills in Lightroom. Just downloaded the trial of Lightroom 4 and it's pretty awesome - seems like a reduced-scale Photoshop with only some key things you really need, and without the mountain of things to learn. Pretty nice. After Peru and Argentina I think I am gonna spend a good amount of time post-processing the best images of that trip, and also going back to some old photos and pimpin' them out a bit. I'll then re-post those to my website, and perhaps split up the photo section into different categories - trimming the images down further and focusing even further on quality over quantity.