EV's "standard" supporting targets AFAIK are 90 days after release of a product.
(Of course this may vary, and depends on service pack schedules, and complexity of the "new" product/sp)
If Exchange 2010 is going to be released in January, and there will be a Service Pack in February and May, then i'd guess it would be May.
(not real dates, only to illustrate... *g*)
/Michel