Looks like whatever it is, it is temporary roll out for testing.
steve40
3:35 pm on May 26, 2010 (gmt 0)
it p?/? me off when it happened to me and I will be very unhappy if that becomes the standard,
tedster
6:07 pm on May 26, 2010 (gmt 0)
A Google employee named Jaime replied:
Update: This is indeed a bug affecting Internet Explorer 8 users. We have a fix and are in the process of rolling it out, so you should have Back button functionality restored later today (within a few hours if all goes well).