I have noticed that when using IE 7, on two different systems (one a dell laptop with XP SP2, the other an iMac dual-booting Vista) PopFly works fine for about 15 mins and then will totally freeze my system and force a hard boot. The freeze happens in the mashup creator, but is inconsistent in that it's not a particular block that freezes.

I've yet to experience this behavior in Firefox however.

-Keith