I dont blame you guys for giving up. Iam not going to buy another steam game I will buy retail games because trainers always work find with them
In the case for Left 4 Dead 2, I bought mine at a retail location and it requires that you activate it via Steam (and I double checked the box just now.) So that would not work for this particular game. What I found is that the Steam options for the game (Properties, then Updates tab) you can turn the automatic updates off and then play with the trainer that matches your current build.
However I do NOT agree with the whole giving up thing. The whole reason I signed up and payed the fee was that I wanted trainers for newly released games. In the case of L4D2, it was released 11/17/09 so I view that as a fairly new game and I really think it should be updated as required. I understand it is not easy to do, and there does have to be a point where enough is enough - but calling it quits before the game even hits a month? Doesn't sound right to me.