I had that happen in InDesign CS5.5 recently. Supposedly, InDesign is supposed to generate a Unique Identifier for an EPUB file if you don't supply one. But it did not. It is required to pass validation for EPUB 2.
You can insert an ISBN number (which I did, and which worked). Or, somewhere I read that you can find unique ID generators. Do a Google search on "generate unique ID".
What Steve said.
Really, it's not that big a deal. Do it once...it's sticky.
OK. I'll give it a try.