Aside from technical difficulties mentioned in @mookamafoob's answer, there potentially is this to consider: Many users in countries that have non-ascii characters in their alphabets have learned that URL's can not be made up of anything but ascii characters. Using identifiers, even if technically possible, might cause users to wonder, if an address they enter really works, or if they should themselves somehow "attempt to convert" the identifier to ascii.
I don't have any official research to back this up, but can distinctly remember a very convinced client telling me that we can not use "äöå" in the folder structure of a website we were developing. The technical reality simply does not always match users' expectations.
Thus my advice would be to stay clear of localized URIs for the time being. At the very least, map ascii version of your URIs to the localized URIs, if you decide use them (i.e. www.foobar.com/aoa redirects to www.foobar.com/äöå, just to be sure that users trying to "guess" an ascii URI find the right page).