As part of the Beta 2 evaluation process, I've seen a couple of reports of customers having problems with Sharepoint log files growing out of control - gigabytes in just a few minutes.  Needless to say, it fills up disks and creates all kinds of havoc.  While I don't know the root cause, we have found a bunch of useful information about it and a solution to make it stop.  I've included that info here in hopes of making it more readily available.

All the cases I've seen so far fill the log file with messages starting with "The previous instance of the timer job

Here's a forum thread on the issue: 

 And here's a blog post on how to fix it:

Hope this is helpful,