I'm a student and programming newbie...please don't flame me.
Yep, the simplest 08 will do it. However, I think the temperature stability of the oscillator isn't brilliant so it would drift.
100Hz (at equal mark/space) is 5mS low, 5mS high, so the simple program below would nearly do the trick, but would be slightly under 100Hz due to the switching and looping overhead (plus / minus whatever temperature drift there is).
Using one of the faster PicAxes with an external resonator would give you better stability and allow you to fine-tune the frequency. How accurate / stable do you need it?