EDIT: Also, I'd like a way to send a message to the person I'm requesting to play with. I checked to see what the message was like and for someone who doesn't know what it's about might be confused, even consider it spam.
As for the message: interesting. It says, effectively: "Someone started a game of Go with you; click here to play." Is that really so confusing? What would you say instead? Do you want control of subject, message contents, both?
A simple textbox containing some optional text to be sent along with the email to provide some context would be nice.
As for the message itself, maybe adding some of the game details such as board size and each player's colors? For someone who knows Go this would make the email seem more legitimate. And/Or you could add a "What's this" at the email's bottom that's only added to game invites that briefly described your app.
Damnit, I know Python, time to check out the code and stop writing lists of stuff you should do!
I wrote more about "weekend hacks" here: http://davepeck.org/2009/01/26/weekend-hack-1-go/
The project is an open-source AppEngine application. The source code is up on GitHub; it is released under the AGPLv3. This was a hack, so don't expect the world's finest code base! http://github.com/davepeck/appengine-go/tree/master
Please send your suggestions (or, better, patches) my way. Happy Go!