Well, looks like that hundred and five bucks wasn't a total loss. I partially disassembled my EF 85 f/1.8 this evening to fish a large dust fleck out of the innermost element group and clean some dust-corrupted lubricant out of the USM focus wheel. I was able to unscrew, pop, and snap apart, and then put back together again, in less than thirty minutes. And it still worked fine afterwards.
Meanwhile, I stopped by a local camera shop and asked about the Tamron 17-50 f/2.8. Turns out they did have one of these
in stock, same basic thing but with vibration compensation, and handed it to me on a 7D body. Conclusion? My Canon EF 28-135 feels like a badly-made toy next to this thing, and I will beg, borrow, or steal to get one of the two Tamrons. Other than seeing the zoom barrel travel outside the body, I could have been holding an 'L' lens. Canon can go eat rocks for charging over a grand for the EF-S 17-55 f/2.8 IS USM.
He who laughs last, laughs first next time.