Nope... it's not a bad copy... I have recently experienced the same issue with the SP AF28-105mm f/2.8 LD (non-Di) and came up seriously soft with my EOS 20D. I ran the lens over to the local repair center and was told the very same thing. Film... it's great... digital... its crap. This is not a cheap lens... it was a mainstay for Tamron in the film market and still is. My EOS-3 has no problem with it at all.
They are currently replacing the circuitry in it to try and adapt it to the focusing of the digital, but that seems to be merely an attempt and not proven. The XR glass on the Di lenses seems to be the main change in these lenses.
Looks like if you want to use an APS-C sensor digital camera (ie, EOS 20D/30D, Rebel, XT), when it comes to Tamron... its a Di or Di II certified lens or suffer softness without end.
That is all...