My sisters boyfriend has the Tamron for his Canon 400, its a very nice lens, particularly for the price. I suspect we are both refering to the same one and I just got the specs slightly wrong....yup looking at SRS Microsystems it does seem to be 70-300 not 75 as I had thought.
Sigma have recently started doing their own optical stabilisation system but I dont think its on any of their cheaper lenses yet. Just one of those specs Im very glad I dont have to think about, since my Pentax takes care of the stabilisation in the body.