The recent rewording of the interpreted languages clause of the App Store agreement ought to allow CouchDB to run embedded in an application.
I hacked on it a bit at Open-Source Bridge last month; didn't even finish getting Erlang compiled. It's all gravy until you get to the bootstrapping step, which may in fact require a jailbreak, or much more Erlang-fu than I have.
And unless every app uses its own couch, you could argue that it's of marginal utility to run CouchDB on the iPhone because of its limited support for "real" multitasking. (You can't use your phone as the couch in your pocket for all your desktop apps, or something like that.) However, CouchDB's focus on replication and the offline use case make it useful as a dead-easy cloud cache and service.