+1 for the Tamron. I own one and have never had anything to complain about. Maybe the focus could be a bit better on my Nikon D80.
I don't think there necessarily is a difference between Made in China and Made in Japan when it comes to their lenses. Nobody here's been to the Tamron factory in Japan and/or China, and nobody here is probably able to compare their production and quality control methods. Everybody knows that Made in China products tend to be of lesser quality of Made in Japan products, but that doesn't mean Tamron products do. I bought mine in Japan, but I've read a lot of good reviews from people everywhere outside Japan before bying it, and I doubt that it would be getting good reviews if the lenses for foreign markets were sub-standard. Fact is, the Tamron 17-50mm f2.8 is good lens, wherever you buy it.