Now, I can see if this is a little too much movie for most people to handle, but I can't express enough how good a deal it is. Plus, you could always leave before Sunshine (I would; I hate it), if you fear a numb butt (try to imagine 24 hours!). Considering I still need to see two of the nominees, it would be worth it for me to attend the whole marathon since at the participating megaplex in Manhattan two movies and concessions would run me about $30 anyway (unless I theater-hopped).
As long-time readers know, I am very glad to see special programming and promotions in the big cinema chains, and though I guess this one depends on how much you like the nominees, AMC regularly has some good offers (such as last summer's free movies). Make sure to take advantage of this or others, or AMC may not see the worth in doing more stuff like it in the future.
The order of the marathon is: Babel - The Queen - The Departed - Letters From Iwo Jima - Little Miss Sunshine. Participating theatres are listed here.