Offer poster printing
If OfficeMax can do it, and at least one pro lab with which I have worked can do it, why not SmugMug, right? 16x20, 18x24, 24x36. (That unnamed lab is only doing 16x20, but if you're going to do it... DO IT!) It would be a great product for those doing photography for kid's sports, and a fantastic product for digital artists.
The real advantage for SM to do this? Since we get to set our own prices in SM, it would be WAY better than some of the other options that either overcharge or pay a measly, preset commission...
Have you looked at the cool options from SmugMug/MyCanvas? http://www.smugmug.com/prints/catalog/P#More
Kimberly Benfield commented
I agree. I would love to offer my clients the ability to purchase cheap posters. Instead, I have to use Costco or Walmart and handle all of the money and deliveries myself. Exactly what I prefer Smugmug to handle for me.
Joe Salas commented
those are just large prints, no? what about proper posters? the kind on cheap, thin paper? the kind where you can buy em at wal-mart of your favorite super hero for $5?