I can't believe this got past me: I didn’t even find out about it until yesterday. It makes me feel a bit stupid, since I wrote this just a few days after Team Fortress 2 received an update on the 360. It's not a complete fix of the networking problems, but it's a decided improvement. I've played around ten full matches – all with sixteen players, the maximum on the 360 – and only two suffered from noticeable lag.
Thank you, Valve, for not forgetting about the 360. I'm certainly less worried about how Left 4 Dead is going to play on consoles now. I prefer playing TF2 – along with anything else that isn't an RTS – on the 360, and it's nice to log onto a match without having to worry about lag every single time. Also, while the game is steadily evolving on the PC, with some big changes on the horizon, the slower pace of the 360 updates guarantees that the original TF2 gameplay will stick around for some time to come.
Some new levels would be nice, though.