If I am to maintain my current levels of productivity and balance in my life, I do not need a tower defense game on my iPhone. But if I *were* to bring such a thing into my life, Fieldrunners looks like a good candidate. I can't wait until playing video games falls under the rubric of parenting. (Just kidding, Meg.)
Also, after a long period with no activity, Desktop Tower Defense is set to be updated soon (hopefully):
Version 1.9 announced! I am working on an updated version DTD which will include multiplayer, extra modes and extra creeps. It will be released in the next few weeks so stay tuned!
But they have a lot of other games under development so I'm not holding my breath.
Update: DTD 1.9 is available here. (thx, christopher & jason)