The bottom end plastic MacBook has just gotten a stealth upgrade — its CPU is now 2.13GHz (i.e. faster than the entry-level unibody Macbook) and it sports a 9400M, so basically if you don’t crave aluminum enclosures it’s simply better than its considerably more expensive sibling.
The obvious conclusion is that Apple will be making fairly major announcements concerning the unibody Macbooks at WWDC. (Apple has been known to launch consumer — or at least non-developer — products at WWDC.)
In any event, the plastic 13″ Macbook is now pretty much the best deal in the Apple universe (if you add a display and keyboard to a Mac Mini you’ll probably end up paying just as much and not have a portable device).