I thought it would be handy to share the following:
Yesterday I tested the upgrade from WP 2.5 to WP 2.5.1. It didn't work properly, especially the media upload flash-thingy. I had encountered this problem before with 2.5
Since I read at this forum that the most common reason for malfunctions is a faulty upload, I tested two different FTP clients; Mozilla and Core FTP.
It turns out, that the latter client, Core FTP, solves all the problems. Apparently, uploading the 2.5.1 files with Mozilla doesn't upload everything required. I tested this with 4 blogsites I have running under WP, and it is indeed what makes the difference between a working environment and a faulty one.
My advice, therefore, would be: use another than Mozilla FTP client if you experience problems upgrading to WP 2.5.1.
I must add, however, that I have no clue whatsoever as to why this is. It is merely an observation and a result of my tests.