Having already done a multiplayer demo for both Xbox 360 and PC and a single-player demo for the latter, Ubisoft has completed the set by uploading a single-player demo for Splinter Cell: Double Agent to Xbox Live Marketplace.
It's available everywhere except Asia, for some reason, but, yes, it's exclusive to Xbox Live Gold members (for around seven days, we're told). Oh, and it's 505MB in size.
For more on Double Agent, be sure to check out Kristan's lengthy review, where he no doubt glosses over the fact that Metal Gear Solid si r0x0r.
And don't forget the other demos, because if you don't believe in them, they won't bring you presents at Christmas.