This is a great sheet, for n00bs and pros alike; I grabbed a copy as soon as I saw it first posted. I do have a couple of suggestions:
1. Post your source files; post it as a ZIP (or if done in photoshop or something, a PSD or whatnot) - put it in the playground, it would allow others to edit/update it.
2. Darken (or remove, and put somewhere else?) the logo on top of the chip; or change the pin number color - pin 23 blends in (a couple of the other pins, 20 & 21, have a similar, but not as bad issue).
3. Where is says "ATMega168 Pin Mapping" put "ATMega8/168/328 Pin Mapping" - small, but useful detail.
4. Increase the resolution of the image (make it huge, or make it vector based, something).
It looks great otherwise; I think with other people's suggestions implemented where you have room, it will turn out to be one awesome sheet (it already pretty much is!)...