Did you move servers after you did an install? (Or did the server configuration change?) The install checks that requirement too, so you obviously passed it then. It's important for avatars and attachments.
I haven't moved servers and haven't changed the configuration. All versions of XenForo have installed just fine, up to 1.01 which I did the day it came out. I'm using godaddy for hosting, so I suppose it's possible they have changed something on their end, although it says I have "PHP 5.x" enabled there (and there isn't anything I can do to change or configure that anyway).