I'm pretty sure the whole thing is built on the JVM, but I agree with you that having a peer run inside a js browser app via clojurescript would be a logical next step. (and arguably really useful)

From FAQ:

Is Datomic just for JVM languages? At the moment, yes. We have ideas for how to enable Datomic on non-JVM languages while preserving as much of the embedded power as possible.

That would be freaking awesome.

However, my big concern here would be security. You'd need to be able to supply a predicate for which datums are allowed to be synced to the client.

Damn, I misread webdev as webdav.