Some things to wait for--but not ALL of them will happen:
* Leopard
* Next iLife bundle
* LED backlighting (but may only be practical for smaller screens)
* Flash storage for less reliance on the HD
* Penryn 45nm chips from Intel (faster, less power/heat)
* Santa Rosa platform (the chips that connect to the CPU--and can be a bottleneck)
* Higher GHz (of course)
* Newer/better video boards
* Completely updated case design (will happen eventually, but the current design has the advantage of being well tested)
* Summer iPod promos (often discounted with a Mac--for students, anyway)
* More selection of models (new smaller size? not important if you're after big)
* Updated non-Pro MacBooks with improved 3D performance (making them a more tempting alternative to a Pro)
* MacBook RED (just possible for 13" or Pro 15", but very unlikely at the 17" size)
* Multitouch display (highly unlikely and of dubious value--although the iPhone technology could lead to fancier trackpads in some way)
Some could happen any time, some probably not until fall if ever... and June seems likely for Leopard and the next iLife.