I was researching this today and this summarizes it quite nicely.
Thanks to Brian K at MSFT for this tidbit on Team Resources and Timesheets for PS207.

For Team Resource assignments to show up on a timesheet – a member of the team would FIRST need to go into My Tasks and add it to ‘take ownership’ of the task.  Then that task would appear on their timesheet.

For those that are trying to make a Team Resource assignment automatically appear on a Timesheet – it appears this is not possible in 2007.  If you need it to show up on the timesheet (via prepopulation) you would need to assign the task to each member of the team.

Also check out my post on Surrogate Timesheets for non-users on PS07.
Enjoy.

-Jacques