The OGC Web Services Common specification refers to a new URN format which has to be used in all capabilities documents:
The above document states, at page 57:
"The OGC URN "urn:ogc:def:crs:OGC:1.3:CRS:84" is here used to refer to the "WGS 84 longitude-latitude" CRS specified in Subclause B.3 of WMS 1.3, previously referenced as "CRS:84".
The trouble is that once defined it can be used for requests as well. For the moment I can workaround in Geoserver by recognizing the code and replacing it with epsg:4326, but it would
be better to avoid these in the long run.