I'll cast one vote in the other direction. I recently made the same decision and went ahead and bought a new monitor.
I agree - prices are coming down steadily, but they are coming down at a linear pace, rather than exponential. By that, I mean that prices are declining, but there isn't some lurch down coming up, just a continued slow and steady decline. Absent some new technological advance, which no one expects, then prices won't have any large move down.
So, it really depends on how much you want one and what it's worth to you. I upgraded from a 17" to a 22" widescreen - the Samsung 226BW. With the $50 rebate (which I believe is still available), it ends up at about $275. That's less that I paid for my 17" Viewsonic back when I bought it.
So, if you're waiting for prices to drop 20%... you're going to be waiting a while. If you're waiting for prices to drop 5%, then sure, wait for a Christmas sale.