How can I use su again after OTA-upgrading my rooted phone?
In short, the OTA upgrade clobbered your root settings, am afraid to say you will have to run either SuperOneClick or zergRush to get root again. 

[TECHNICAL]

The OTA reverted the original default.prop when on Froyo from this

ro.secure=0 


to this after the upgrade

ro.secure=1


Thus you may still have the su binary etc, but it is the above line that is preventing you from trying to switch user to id of 0 (that's what su stands for! - S witch U ser)