As far as the current released Windows Runtime API is concerned - there is no database engine available. That might change but currently we need a way to store local data.
Its more or less the same situation as we had on WP7 before Mango.
I think porting it to WinRT is not a really big issue as many .Net APIs are still there. Something changed in handling storage (everything is async right now) but the Sterling architecture is rather flexible if I remember right.
Any short term plans for porting Sterling to WinRT?