This is usually frowned upon by the W3C Tag... as I know from experience on the XRI TC... becuase XRIs use an alternate resolution process. However I've created shortxri.net which shortens URL's to an XRI, which can then be used as relative URLs. Now one can tack an XRI on any domain which supports XRI resolution, and you can push the XRDS (which the XRI ultimately points to) to other domains as well. So the xri @si348u can be attached to http://shortxri.net/@si*3*48u or http://xri.net/@si*3*48u or the even shorter http://xri.be/@si*3*48u to all resolve to the same place.

XRIs don't even have to be just URLs... and logging in with OpenID gives you even more options.