I am a heavy Toodledo user on my iPhone. At the same time I use Google Apps for my domain, so I am a Gmail fan too.
You might be aware that Google recently launched a service for iPhone and other mobile users - Google Sync. People can sync their Gmail contacts and Calendar to iPhone.
So I thought why not try Toodledo to sync my tasks to Gmail Calendar and hence in turn try the iphone calendar to have the tasks I add to my Ultimate Todo's application on iphone.
To do this I did some tricks and made it work. Thanks to the Live iCal service Toodledo provides and Google Sync.