1.  First, setup a free task-tracking account with Remember the Milk (RTM):

http://www.rememberthemilk.com/

2.  Next, add the RTM task pane to your Gmail.  It's in the "Labs" section of the Gmail Settings area.

3.  Log back in to your RTM account and LINK your Google account to your RTM account in the RTM Settings section.  This will give Google permission to access your RTM tasks and account.  I

4.  Next, subscribe to your RTM Calendar as a Remote/Network iCal in Thunderbird.  The format is

webcal://www.rememberthemilk.com/icalendar/(RTMusername)

5.  Voila! The RTM Tasks come along for the ride along with the RTM Calendar and show up OK in my Thunderbird Tasks now...since RTM and Google tasks are always synched, they are one and the same.

It's not perfect, and there are some bugs, but it's a good start.  Perhaps some of you can help figure out how to improve this technique.

BY THE WAY:  I use Astrid as the Task manager on my Android cell phone, and it synchs with RTM.  So everything is totally synched...my phone tasks in Astrid, my tasks in RTM and Google, and my tasks in Thunderbird's Task pane...

Took a while to come up with this, but it seems to work somewhat well.