Major reason for 2.5 was Windows Live OneCare for Server
As for minor fixes and updates, they are happening all the time and even if they did bother to post them they'd likely simply create confusion. If you want this type of inside information about your protection products you'd be better off with a different product. OneCare is designed for those who just want to be protected and don't care to know such information.
Other than major issues which have been fixed in a recently released version, I've never seen a listing of the minor changes that have occured. It used to bother me too, until I realized it was simply curiosity and not anything of real value that caused me to be interested.
In fact, my relatives who are the main reason I became involved in OneCare could care less and they are precisely the target for the product. In reality they are better protected by not being required to know any of this information, since all it really does is confuse and scare them anyway. The "set-it-and-forget-it" nature of the product actually makes it useable by the non-techno-geeks it was intended to serve.
OneCareBear