If you are using hosted (virtual) domains, all WCAP commands you issue must have fully qualified user ID's (uid) and calendar ID's (calid), for example firstname.lastname@example.org.
In order to be in hosted domain mode, several parameters in the ics.conf file must be configured as specified in chapter 12 in the Calendar Server Administration Guide: Sun Java System Calendar Server 6 2005Q4 Administration Guide.
See your Calendar Server administrator if you do not know whether you are using hosted domains.
The following two example WCAP commands demonstrate the difference between calid values for non-hosted domain mode and hosted domain mode.
Non-hosted domain mode:
http://webcalendarserver/get_userprefs.wcap ?id=b5q2o8ve2rk02nv9t6 &calid=jdoe &fmt-out=text/calendar
In hosted domain mode:
http://webcalendarserver/get_userprefs.wcap ?id=b5q2o8ve2rk02nv9t6 &email@example.com &fmt-out=text/calendar