Jess: You like low-energy hardware? Why? RISC OS isn't unique in running on low-energy hardware, and you won't save much given the premium RISC OS kit costs. If you're thinking about the environment rather than you pocket, then RISC OS is also a lose: the larger a manufacture run, the less the overhead and pollution. Additionally, your ugly idea of having another computer running apps RISC OS can't for you doesn't seem very energy-efficent!
Thin clients are a completely different kettle of fish. They don't tend to have much GUI of their own, they're for running remotely managed applications. Having a RISC OS desktop with Messenger, Zap, ArtWorks etc, then then having a big fat window with OpenOffice running in it doesn't sound nice to use at all - and it won't integrate nicely like CrossOver or WinRisc (if "nicely" is used very losely.)