Extreme simplicity is good for mobile applications. While your app goes a bit too far, it's a good direction. A slightly less minimalist mobile version with the ability to send invites by SMS and mail might be very useful, though that application overlaps with Twitter. It would be better for the specific purpose than Twitter, however.

Imagine:

    Hey guys, how about checking out this new joint for a beer?
    http://mobilvite.com/i/abcdefghi

They click on the URL and their phone goes to a nicely designed browser page with the details, which has been populated by the host-user filling out a few items like where, when, in a simple form. The Javascript on the form can detect if it's on an iPhone or other smartphone and present a directions button that pops up Google Maps.

The advantage of this over Twitter: the information is presented to the invitees cleanly, and they get the details only if they ask for it. Sending multiple tweets or multiple multicast SMS would be cumbersome to compose and annoying to receive.