I think an appropriate date for the MBs to be updated would be in late August, or the first week of september. The last generation of MacBooks (using the Santa Rosa platform and Penryn processors) were released on the 26th of February, while the actual Penryn Processors were released on the 6th of January, i.e. the MacBooks were updated a month and 2 weeks after the processor came out to market. The upcoming Montevina platform is scheduled (rather, delayed) to come out on the July 14th, along [verification needed] with the new Penryn Processors T9400 and the T9600 (2.53Ghz and 2.8Ghz respectively, both having 6MB cache, in contrast with the current 3MB cache in the MBs). So, in my opinion, Apple will update the current line of MacBooks with these configurations, which will take Apple about a month and a half to update. Well, this is my 2 cents.
Btw, I didn't consider the student promos etc so Apple might have a plan to release these cute white and black beasties later on after the promo this year.