You might also want to consider the non-VC version of the Tamron. I've had one for just over 2 years (on a 450D and now a 50D) and have been delighted with the results.
VC or non VC tamron???
Isn't the VC version an improvement?! Has it made it that much difference?
On amazon the non VC is only £295 I think which is a great buy!
how about the sigma 17 - 50 f2.8 OS?