Jointly with ISWC organisation, we have negotiated special conference rates for RR 2008 attendees. In the Renaissance Hotel Karlsruhe, rooms have been blocked explicitly for the RR weekend.
The offers are valid until 4 weeks prior to the conference, so you should reserve fast. The joint booking form for RR2008 and ISWC can be found here.
People visiting both events can do the booking on one form.
You will have to fill it out (can be done electronically within the PDF), print it, sign it and fax it back to the number indicated on the form.
For individual hotel booking options see e.g. this Website.