Thanks everyone for the suggestions. I'll check out the Basecamp sync to see what it can do. I also agree that Toodledo doesn't have the prettiest GUI, but it works and it has well-published API that makes syncing adapters easy to develop. In my opinion, any software application that wants to see long-term success in today's "web 2.0" world MUST be fully interoperable and should either maintain APIs or make them easy to develop.
What would be really cool is if someone started an open standards group to define the most common metadata associated with personal task management (i.e., contexts, project categories, due dates, sub-tasks, etc). If it were well-planned and marketed properly, all the task software apps (like Omnifocus, Toodledo, Things, RTM, Outlook, etc) might be compelled to write an API to interoperate with such a standard.
There has been some success with this regarding calendars: iCalendar/CalDAV and
RFC 5545. Our reasonable (though not yet perfected) ability to sync Google, Yahoo, iCal, and Outlook calendars is due to the work done here; when will someone step up to the plate and do the same with tasks/to-do's? Probable answer: when someone sees a good business opportunity for it!