Logic would dictate that with the move to all LEDs in their portables and the introduction of a LED display, Apple will be phasing out all LCDs in favor of LEDs as products get updates (the iMac, Cinema Display and white MacBook being the only non-LEDs currently shipping).
That being said, based on past years, it's unlikely Apple will have any major updates before Macworld at the beginning of 2009. They traditionally don't update for the holiday season.
If anything, there may be a very minor revision to the iMac before then (say, a processor speed bump) but no major change like LEDs.