Toodledo is a web-based todo list manager that includes a dedicated iPhone app, as well as a regular mobile web interface for non-Apple phones. A third-party developer has used their API to build an Android app for the G1 users out there. They offer a very functional free option, or a "Pro" account for ~$15/year. On the PC end, you can obviously interact through your browser, or there is an Adobe AIR app, and gadgets for the Apple Dashboard and Vista sidebar.

Their setup works well within the context of GTD as the place to keep all of your lists. They have linkages with Twitter, Jott, Google Calendar, Firefox, and RSS feeds, plus you can always dump your data out to a CSV and take it somewhere else if you want to leave - your data isn't held hostage.

I've been using them since November and I've been pretty happy with the service.