I tried out the poster printing service from PosterBrain.com last week and printed out a couple copies of The Caffeine Poster. I've never posted the full, high-resolution image online, but I used it with PosterBrain to print it out fullsize 24" x 36". (Apologies for my not-quite-professional photos)
The poster is gorgeous! I'm so impressed by the PosterBrain service, and the quality of their prints.
You really should try their service. I uploaded the file on Sunday, and paid only $24.99 each to have them printed with free shipping. I received the posters on Friday by standard USPS mail. They were beautifully packaged with sheets of tissue separating the posters.