I'd love to fit an SSD in my iBook and Cube, but I'm with peeb on this one. I think it's way too expensive for the moderate performance gains you would see. Once it costs ~$150 then it might be worth it, but until then, I seriously doubt the (very slight) increase in read-speeds would justify it.
As for battery-life, it would appear that SSD doesn't really give you a net gain in that regard--
at least with the MacBook Air, anyway. I imagine it would apply to SSDs more generally.