http://www.nowgamer.com/news/5030/portal-2-mod-levels-for-all-platforms
This includes Xbox 360. Now I can finally make my brother think with portals! 
Custom Content on Consoles CONFIRMED (lol Alliteration)
652 Posts
Posted Jan 24, 2011
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1,366 Posts
Posted Jan 24, 2011
Replied
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I'm wondering how Valve will let 360 users access custom content. Maybe DLC? I'd really like to see a Valve-hosted contest where the winners get their maps featured as DLC.