Um.... You obviously have not see a new 22" lcd have you? They look fantastic, why would you use a 15" that is so IBM PS/2 size. Get with the program, you aren't shopping for a car waiting for the Hybrids to get cheaper and really take off. It's just a monitor. Sure there is new technology coming out in the furture but when is there not.
So to answer your question... YES! I would buy one now. I went from a 17" to a 22" and you never want to go bat to those tiny little displays, it sucks gaming and even for just general desktop work.
Yeah the 20 and 22" displays have the 1680x1050 res. But 22 is still bigger. It's like going 17" to 19" same res just bigger. Right the oLED screens will be thinner, brighter, prettier, but $$$$$$$$,$$$$$$$$. I remember getting my first 17" LCD at $500. An they were out a while. Good luck waiting.
Get a 24" mva monitor, a friend of mine got himself a Westinghouse L2410NM 24" LCD for about $400 shipped and I can tell you it's worth every penny.
The only downside to those is the requirements for gaming on it at native res(1920x1200), small sacrifice for great quality if you ask me.
15'!! that barely registers as a monitor. It would be like playing all ur fav games on ur iPod. You can always wait for whats round the corner, if its very close it might be a good idea, but 2 years is a very long time staring into a 15' monitor. By which time u will probably be blind from having to strain your eyes.
Modern lcd monitors are very nice, and size does matter. Buy at least a 19' or else you will regret it in time.
Obviously $$ is a factor for the OP or he'd likely not be posting this question that seems so obvious to the rest of us.
If you can afford to get a nice 22" today I'd say go for it. If you can stretch for a Samsung 245BW (nice, but not too nice...and not too expensive), that may be even better. However, I would buy a great 22" before I bought a so-so 24". I think a 20" @ 1680x1050 may feel a little cramped before you know it and you'd be looking to upgrade.