Read this thread: Tasks Bug on 8800 - Dates Are Transposed One Day Into The Past
I noticed the same thing when I first switched over to BlackBerry from my Treo, and after a long time working on it, conclusively determined what the problem was. It is all described in the referenced thread.
This is a bug related to how Oultook handles tasks and how the BES/BlackBerry handles them. There is no solution to it that anyone is aware of.
Suppossdly, RIM is working on this, but it was brought to their attention a LONG time ago, and so far, no response/fix.