Testing WordPress 2.6 beta 1 on the this blog.
Upgrade worked well; no major issues so far.
One point to note – publishing via XML-RPC (i.e using a blogging client such as Windows Live Writer) is disabled by default. You have to turn it on.
If you don’t then WLW throws an error:
Server Error –32601 Occurred server error. requested method blogger.getUsersBlogs does not exist