Where it stays for hours and hours. It looks like it is actually stuck on a lock in the database.
We upgraded another 6.0 instance and had this problem. HOwever, we obviously cannot upgrade production until this is resolved.
We have tried the test instance twice now and gotten the same result. The first time we saw NullPointerExceptions and ArrayOutOfBoundsIndexExceptions which suggets to me that it failed the first time without reporting the exception.
This time it is running with no errors but appears to be stuck again. Specifically it looks like it the DB has locks on a few WP_ tables.