I was having issues upgrading the site to PHP 5.1, mainly because it broke a number of the GrabPERF graphs.
Then I read the PHP docs. Turns out that there have been some internal changes to the strtotime() function. I had been using strtotime(date(Ymd H), time()) in the broken graphs; this now returns FALSE, not a numeric value.
A simple switch to strtotime(date(YmdH0000), time()) resolved the problem entirely.
If you see any weirdness, let me know!