I was having problems on a production site, but I've installed the current version of the All-in-One Event Calendar plugin on a test installation of WP 3.4.2 with no other plugins installed.
After creating a couple of events, I went to the calendar page and clicked the
"Subscribe in Google Calendar" button. The first time I tried it, I got
The address that you provided did not contain a calendar in a valid iCal or GData format.
(the error I'd been seeing in the production site).
When I tried it again, there was no error message, but the events from the WordPress site aren't showing up in the Google calendar.