I'm blanking, being absolutely sure I've come across this before on this list, but can't find any references to it. Is there a command available (from ruby, ideally) to refresh a document, based on changes made to the associated disk file?

TextMate will reload the file when it gains focus, so all you would need to do is click on the finder then back into TextMate. If you want a command to do it just create and Applescript to do the same.