What do I need to get from a webhoster if I want to host my own Diaspora* seed? Do I just need some webspace with ruby on rails capabilities? I suppose it uses Ruby On Rails SOAP for communication between seeds? If so what is needed for this?
According to the README on GitHub, you need Ruby, MongoDB, OpenSSL, ImageMagick, Git, RubyGems, and Bundler... so any decent shared hosting that allows you to build things in your homedir and run custom long-running processes should do the trick.
(Decent because the non-decent ones may omit Git or ImageMagick and not give you the resources to install them yourself. Long-running processes for MongoDB and the Diaspora app server.)
WebFaction, for example, should meet those needs but Dreamhost may not unless the Diaspora app server can somehow be managed by FastCGI to work around their long-running process killer. (Dreamhost apparently supports MongoDB, so that would be exempt from the killer)
Seriously though, I'd recommend waiting. Diaspora was just released. It's still in development and, if anything, probably best to be experimented with on your home PC for at least a few more months.