Jedi-Bird sends in the following:

Just noticed something interesting in AOTC. Jango Fett's Mandalorian helmet is in pristine condition until fighting with Obi-Wan on Kamino. After the scuffle Jango Fett's helmet has a dent on the left side (from Obi-Wan's kick or maybe the headbutt from Jango?).

This dent can be clearly seen while Jango Fett is holding the helmet behind Count Dooku on Geonosis. Even after Boba repaints his father's helmet green, the dent remains (as seen in ROTJ). This is also reflected in the new action figures. No dent in the Jango Fett- Kamino Escape but dent present in Jango Fett- Final Duel.

Update: Some speculation on the origins of the dent. The first from Joe:

Plenty of people have probably already realized this, but here goes: One of the news things up right now mentions the dent in Jango's helmet which appears only after Kamino. Well, I assume it happens when he hits his head as he goes up Slave I's ramp in homage to the accidental stormtrooper head-bump in A New Hope (something I noticed at the theater and confirmed by a recent homing beacon from starwars.com). May the Force be with you!

Quest adds the following:

Jango gets the dent while boarding Slave 1- the ship's door closes and bumps him on the head (you can hear the "clank!" noise it makes when it hits him), and while some debate that he actually hit his head (despite the obvious sound effect evidence), his helmet does not have the dent when he is climbing back up the edge of the landing pad, and yet it is there on Geonosis, so that has to be why the dent is there.

From Zartan Fett:

The dent in the Kamino battle with Obi-Wan is actually on the top right hand side in the middle, not towards the front like Boba's. The dent occurs when Jango lands on his head over the edge right before he pulls Obi-Wan down with him. The dent which we are supposed to be believe is the same dent as Boba's is the one Jango sustains on Geonosis after being hit and run over by the Reek. This dent is in about the right spot as Boba's on the left towards the front and we can only believe that Boba keeps the dent to honor his father Jango who fell to Mace Windu. It should also explain the Padawan Braids that Boba carries in the later episodes. (correction: the braids Boba Fett collects are wookiee scalps)

Thanks to everyone else who wrote in!