How are you guys planning to get traction? This is a really crowded field, and adoption doesn't appear to be driven by rational behavior. For example, Invitastic (http://invitastic.com) is already a pretty darned good replacement for Evite, but nobody knows about it (no personal association; they just happen to be a local company in the space).
Also, a few seconds of googling turned up this laundry list of pretenders to the throne:
http://www.scurrilous.com/blog/archives/2007/03/30/evite-alt...