There is now a Safari extension for Safari 5.1 called Safari Keyword Search by Arne Martin Auerlie. It uses new API calls in 5.1 to reproduce all the functionality of Keywurl without the need for a SIMBL hack!
It is still an early version, so the interface is still a little rough, but it works wonderfully.
Get it here:
Read Arne's blog here:
Also if you don't mind hacks, Safari Omnibar does something similar, attempting to replicate the behaviour of Chrome's Omnibar.
It is a bit hacky, so I would recommend Arne's extension in preference to it, but it is worth watching how both projects develop.