I'm a big fan of gmail and google calendar as my centralized PIM. One major feature is missing however: there is no support for ToDo lists. You can add a simple ToDo google gadget on your iGoogle page, but that todo list is only accessible from within iGoogle; there is no integration with gmail or calendar whatsoever.
Some greasemonkey scripts also add todo lists to google calendar. Unfortunately, they are very sensitive to site fixes and updates: any update can break those nice scripts.
As an alternative, I just signed up to Remember the Milk. 'RtM' provides a rich ToDo list that integrates well in gmail and calendar: