Interesting. Even though it's not officially sanctioned by VALVe, I can't imagine that they would pass up the opportunity to make a browser-based version of TF2 for people like me with shitty computers that wouldn't really run standard TF2 well. I mean, Quake Live pulls it off fine. Connect to a match, watch a commercial, play the game. And you wouldn't have to worry about the Mann-Conomy to make the game profitable; just have commercials for the free players and a premium membership option (with more maps and modes) for the ones who are willing to pay. Again, Quake Live, in my opinion, demonstrates this well. They've been here this long, right?
The only downside would be that it would be another obstacle in the path of them finally releasing Half-Life 2: Episode Three. As if we haven't waited long enough. We're about to start entering Duke Nukem Forever on the waiting schedule. >X-(