Have you seen the Corn Mug?
They're made of 100% corn-based plastic, from US grown corn, which makes it bio-degradable - so you can throw it right into your compost pile (or landfill) and it'll safely degrade (although I failed to inquire about the ink used in printing on the mug...) As an added bonus for those in the good ol U.S. of A., the corn mug is manufactured in Missouri.
I talked to Corn Mug distributor, Dwayne Burnell (who just happens to be based in my home town near Seattle, Bothell, WA) who said these particular mugs (like the thick walled 'Corn Koffee Keg'), with the corn plastic technology has been around for about a year. Burnell said that previous mugs were too thin and couldn't withstand hot liquids. But this one seems to be a winner.
It is microwavable, dishwasher safe, it doesn't chip or crack, and it withstands hot liquids like its regular plastic counterparts. (The only caution of note is that it shouldn't go in commercial grade dishwashers - those are too hot, the plastic bubbles slightly, but the dishwasher in your home is fine. Unless you don't have a dishwasher at all - like me - then there's no need to worry about the dishwasher factor.)
Burnell says Cornmug.com came about in a marketing class he taught. He wanted students to think about product lifecycle - and the corn mug seemed to be a perfect fit. So if you need an eco-friendly promo item with your company logo - go for the cornmug.
Of course there's probably huge drawbacks to corn based plastics (which I hope you will point out to me), but I'm all for them. In my opinion, corn based plastics (and related packaging materials) are significantly better than petroleum based plastics simply because corn based plastics are biodegradable and will breakdown after disposed of, whereas other plastics last for a lifetime (if not longer).