Yes, exactly - FSB wall...
Team Turkiye catched waaaaay better E6750 - http://valid.x86-secret.com/show_oc?id=210128 but judging by some reviews on the Internet of the E6750 the ~4GHz (probably walled too) is the limit of nearly all E6750. Maybe retail will be better.

And this CPU has FSB-wall on air with P5K-Dlx on ~490 (tested not by me) - I don't really know why on Commando (and air, too) it was "simple" 510+.