I haven't been able to find this documented in the iOS manual, but it appears that you can 'shake' to undo following actions: typing text, cut, copy or paste.
It can only undo the immediate most recent action, and doesn't allow 'multiple undo.'
This works in Safari, Mail, Notes, and various apps.
[crarko adds: This has been around since iPhone 3.0, and I was surprised to see we didn't have a previous hint about it. So I'm running it mostly for the sake of completeness.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20101019112648598