...
http://download-west.oracle.com/docs/cd/B13866_02/calendar.904/b10893/part2ws.htm
also:
http://www.tju.cn/docs/ocs9042/calendar.904/b10893/wssoap.htm
http://www.tju.cn/docs/ocs9042/calendar.904/b10893/wsjava.htm
(I found this via Google, sorry I don't know where the official documentation for this is kept!)
So in theory it should be possible to interface an uPortal channel with Oracle Calendar, although you would also have to deal with Oracle's Single Sign On mechanism somehow.
There is also a command line utility called UNIICAL which can be used to import iCAL objects into the calendar server (found inside Oracle Calendar Reference Manual).