Activision's released a single-player Black Ops demo for Xbox Live.
The taster comes in at 916.64Mb. No word on what the exact contents are in the taster, but the synopsis for the demo says:
"Highlights of the mission include reconnaissance in a supersonic Spy Plane, mountain top rappelling and base-jumping, and fighting enemies in stealth mode as well as full firefight mode."
You can queue the demo up from here.
Treyarch confirmed earlier that a new update for the game on 360 has now gone live, with another update for PS3 due to go live alongside the release of the First Strike DLC for the platform on Thursday.