Amazon is now taking pre-orders for the Sigma 17-50 OS announced earlier this year. The price is similar to that of the Tamron 17-50 VC when it first came out (the Tamron has since gone down to around $600 after rebate). I haven't found reviews on the Sigma yet though. Hopefully it's as good as, or better than, the Tamron.
I'll update this post if and when I see any reviews.
Because of the absence of any reviews, it only makes sense to buy it from a source that has a good return and exchange policy, like Amazon.
Note: although the Sigma 17-50 is available for Pentax mounts, it cannot be used concurrently with Pentax's in-body stabilization. The 17-50 is also not compatible with the K100D and *istD.