Todo on the iPhone has been able to sync with online services Remember the Milk and Toodledo for a while now. With the addition of Appigo Sync, however, you can now get your todos from iCal (though, unfortunately, it cannot sync with both your Mac and an online service). Like most other desktop syncing solutions for the iPhone, it works over WiFi and requires your iPhone and Mac to be on the same network.
Appigo Sync is a free download. Todo for iPhone is $9.99; there's also a free Todo Lite limited to seven tasks, but which will allow you to test out the syncing (both iTunes links).