Now that I have my site up and running (fixing small problems with your help), I thought about what's the best way to use a WP/BP site with multiple domains. It doesn't have to provide different content, so actually MU isn't needed. I remember I already tried out some rewrite on the server but this gave me several problems (e.g. with login, address, ...). So, what is your recommendation? Should I migrate from a single install to a network install and use the Domain Mapping plugin to setup the domains? Or is there an easier and full proof method?