Well the thing is that the demo version of XSI 3.0 is now at beta testing stage. It means that it could be released in the future (or maybe not), but I think it would worth to wait for it.
(I'm not on the beta cycle, I just happen to know that it started
)
Usually, evaluation copy of XSI can be found, as Uncletoon pointed, at the developer site.
Hope htis helps
Salutations - Cheers
Bernard Lebel