Quick fix for now is to simply edit the MaxBundleVersion value in the Info.plist of the Keywurl.bundle. Haven't noticed any problems. I've posted some instructions with a few screen captures at http://hacketal.com/making-keywurl-work-with-safari-5
Update: See below for more details but I believe this fix only works for Snow Leopard users. Unfortunately, I do not have a machine with Leopard installed to try and fix it there as well.