Richard, unfortunately this isn't possible, but you might find the keyboard shortcuts useful -- after you press enter to add the task, you can press 'y' to jump to the add note field for that task.

Personally I wish I had some way to make the 'Notes' tab always be the default selected one. Then I would see notes as I mouseover the different tasks. Also, some kind of icon or other visual indicator (maybe italic task title?) that signifies the task has a note would be greatly appreciated.

Certainly not an original idea - a search shows that many others have suggested this - I just wanted to get in yet another vote for making the 'Notes' feature a little more prominently usable.

but if you add a task in a smart list, it disappears from the smart list, so the shortcuts don't work unless you are in a regular list. :(