I just googled this topic to try to find out more; it appears there is already multithreading for TF2.
http://forums.ngemu.com/game-console-discussion/104432-team-fortress-2-multi-core-options-pc.html
Apparently the commands aren't fully developed yet, so maybe that's what you all meant when you said you want to see the TF2 update with multicore support.
If you don't want to read that, here's the wrapup: poster tried it on an AMD system (multicore, don't remember if it was quad or dual core) and reported about double the framerate. Other people in other threads I've read say they've seen little to no improvement, but that's probably (in my opinion as well as theirs) because their GPU is bottlenecking them, not their processor. The poster of the thread I just listed said that he had no instability issues, but some have reported such issues.
Maybe someone here could try out the commands at report back. Here they are:
First, the DEFAULT settings (single-thread):
r_threaded_renderables "0"
mat_queue_mode "-1"
r_threaded_particles "1"
r_threaded_client_shadow_manager "0"
cl_threaded_bone_setup "0"
cl_threaded_client_leaf_system "0"
Second, the multithread (multicore) commands:
r_threaded_renderables "1"
mat_queue_mode "2"
r_threaded_particles "1"
r_threaded_client_shadow_manager "1"
cl_threaded_bone_setup "1"
cl_threaded_client_leaf_system "1"