Be creative, just don't be *too* cartoonish. We may (in the future) have a Newt mascot (for things like the FailWhale), but that doesn't mean the logo has to be a full-grown Newt. We're more inclined to pick a subtler interpretation of the Newt. So, a tap-dancing newt with a tophat and cane is probably going in the wrong direction, but if you can make it work we're willing to go there.
Must be delivered in a vector based format along with a few sample hi-res images.
Should (but it's not a 100% requirement) work at 4 or fewer colors.
Should work on a business card, website, banner ad, etc.
The bottom line: Have fun, but not too much fun.