hmmm... now that you mention it, I did some quick checking on newer versions and indeed the XML logs from Messenger are changed since the new versions of Messenger.
Now they indeed are named as
xxxxyyyy.xml where
xxxx is the first part before the @ symbol, and
yyyy is the .NET passport ID
(not a random number, but a calculated hash based upon the complete .NET Passport, see here)
And in even newer versions there is indeed no mention anymore of the
LogonName attribute inside the
user tag either.
(so you aren't going crazy, nor is your logging system messed up; I didn't checked it out to carefully before replying)
/ 
