The smartphone version of Google Calendar has allowed users to check their schedule for some time, and it has recently acquired a way to respond to invitations and change attendance status on the go.
Those with an Apple iPhone, iPod touch, or T-Mobile G1 just have to select an event to change their attendance status, edit the details, and add or remove guests.
This is one of Google’s new online options that is partially usable even when the phone doesn’t have a connection. It can will still start up and show recently-viewed events.
It is is free service.