There is one additional advantage to buying them together: If you get AppleCare, the warranty will also apply to the monitor. There is, otherwise, no way to get a 3 year warranty on an Apple display, sad as that is.
Considering the price of the 30", I'd find the extra couple years comforting.
Other than that, I don't expect any huge changes in the lineup. iSights are likely at some point, but who knows when (admitedly, the Mac Pro release seems a likely opportunity). The smaller ones might see a price drop, but the 30" is already only a bit more than Dell's 30", so it probably won't get a whole lot cheaper.