We are actively working on a massive XBox 360 update, which will include pretty much everything we've shipped on the PC in the last year. We'll provide you with a rough date when we have one.
We're also working on bringing over a variety of engine improvements from Left 4 Dead into the TF2 engine, but it won't be a wholesale replacement of the engine. This means we'll get some of L4D's performance improvements, like the better multicore support, but not all of them. For example, we can't bring over some improvements without dropping support for DX8, which we're not prepared to do.