Ubisoft's released a PC demo of Rainbow Six Lockdown, allowing you to fight some terrorism before it all gets out of hand and in the shops. The PC version is out in early February, and this 497MB demo lets you play a South African single-player mission.
Also, for those of you who want to fight as some terrorism, presumably, there's a multiplayer Prison map, which can be played in brilliantly named "Team Adversarial" mode. As for the single-player, that's a campaign mission available in two difficulty settings.
Ubisoft also sent over some other stats about the demo. It's got 17 out of the game's 42 weapons (though, er, quite what they are...) and all the equipment and grenades, plus, on the multiplayer side, you can use the Custom Character Creation majig.
It also sounds like it'll work on PCs that you've had from a while, since the minimum specs are a P4 1.5GHz (or AMD equivalent), 512MB RAM and a 64MB DirectX 9-compatible graphics card.