If you don't want to use NSURLConnection directly (it's kind of a pain) there are a few Objective-C frameworks to choose from:
Trundle is being used by a few apps already.
CouchCocoa is a newer framework, somewhat higher-level than Trundle. It's supported by Couchbase, but is free and open-source.
CouchObjC is an older project that doesn't seem to be maintained anymore.
If you want to use CouchDB in an iOS app, with a local database, you'll likely want to use the Mobile Couchbase framework.