One thing that u2 does, is change the format of your User Agent string. 7.54u1 was shown as "Opera/7.54u1" where u2 is now shown as "Opera/7.54" (no u2).
Hester
11:52 am on Feb 7, 2005 (gmt 0)
Apparently that caused problems with some scripts that didn't know what to do with the "u1" bit.
