That is the theory. Does it actually work on the iPhone/iPod touch when there is no connection?
With the requisite warnings about "no connection present" duly dismissed (this is an iPod Touch 2.0, so no AT&T network backstopping it either), I can browse my GMail inbox, read messages, make changes, and perform seemingly any "normal" mail function all without a connection. Obviously will have to connect at some point for it to be useful, but can do everything but send/receive new messages while away from a network connection.