I copied the stuff below from the TSA website. They provide no definition of what a tac pen really is. The "final decision" wording indicates that it depends on which officer frisks you. Has anyone put this to the test, or is it simply not worth the trouble?
Tactical Pen
Carry On Bags: NoChecked Bags: Yes
For more prohibited items, please go to the 'What Can I Bring?' page.
The final decision rests with the TSA officer on whether an item is allowed through the checkpoint.