I switched to Dvorak about 4 years ago due to some pain in my wrists. It took me less than a month to regain my old speed, maybe three weeks. This was the same for everyone I knew who switched cold turkey; people who went back and forth with QWERTY during the transition period took two or three months to become proficient (or gave up). After less than six months, my wrist pain was virtually gone, and has remained gone (disclaimer: I used to play guitar and video games more frequently as well, so I'm not certain this was the only factor).
I had read on a pro-Dvorak site at the time that most people get a 30% speed gain. I was in the lower 70s with QWERTY and am now in the upper 70s, so for me it was more like 10%. Still, over the course of the last 4 years, I've probably made up for the slowness of the month when I switched. Think of it this way: you already know how to type, so switching is essentially rebuilding your muscle memory. Since your fingers will be travelling a shorter distance under Dvorak, once your muscle memory gets to the same point you're at with QWERTY, you'll be faster by definition.
If you're going to switch, my advice would be the following to try to reduce the learning curve:
- Don't switch back and forth until you've got Dvorak down. All major OSes have fairly straightforward ways of switching to Dvorak; learn them and use them, even if you're borrowing a machine.
- Print out a diagram of the key layout and put it up next to your monitor. This is better than relabeling your keyboard, in my opinion, because then you will get out of the habit of looking down to see where you are (which should also improve your speed, even if you decide to go back to QWERTY).
- Try to time the switch around a time when you're going to be using a computer frequently, but not for anything time sensitive. I work in IT, and I timed my switch around a stretch when I knew my primary task was going to be working with numerical data. Be prepared to send a lot of one-word e-mails! If you've got to do a large amount of time-sensitive typing, you'll be inclined to switch back and forth, and that will be problematic.
- There are a number of online keyboarding exercises designed for Dvorak - use them! I actually observed noticable speed gains daily using these. I don't have any of the links any more, but I don't recall them being difficult to find.
- Be wary of keyboard shortcuts. As others have mentioned, many application specific keyboard shortcuts are in part based on the physical location of the key on QWERTY, so depending on what else you do, it could be problematic. (I've gotten used to the cut/copy/paste type functions and many shortcuts in Vi, but there are some applications where I just switch back to QWERTY if I'm not going to be typing words.)
I've become enough of a Dvorak evangelist that it's become a sort of joke among my friends and family. I honestly think it's the best computer-related decision I ever made, and I've gotten a great ROI.