Not in Safari, but there are other browsers like Terra (http://itunes.apple.com/us/app/terra-web-browser-tabs-full/i...) that let you select a User-Agent. Sites that do this are pretty much the only reason I have Terra on my iPad.

You can change it in Safari, but only if you Jailbreak. In fact, it's one of the first apps Cydia recommends to you (I'm using it to identify as desktop Safari, and it's blissful!)