It was there in "stock" 2.2 as well, on the Nexus One --- but disabled in almost all carrier builds. And Google does have some leverage, if they choose to use

It was there in "stock" 2.2 as well, on the Nexus One --- but disabled in almost all carrier builds. And Google does have some leverage, if they choose to use it, in the Market compatibility requirements --- there are minimum requirements for both hardware and software if you want to get the Market, or Google's other proprietary apps. (Which, say, the Nook Color flunks --- no camera.) It might be a bridge too far for some of the carriers, but from the outside, it's not obvious that they're trying. (And looking for other carriers is not a hugely practical option in an environment where there are basically four, and the one with the most generally consumer-friendly policies, T-mobile, is up for buyout by the one with the least, AT&T, which is the only one to ban sideloading altogether.)