Don't think anyone tried that yet... it seems as though most overclocking efforts have been to OC the iPaq 1910. I could be wrong, though.
[edit]
OK, never mind, I was stupid. Anyway, I tried the Speed Stepper program and it WORKS! The program now says my Axim is at 398 MHz, and I now get 7.9 fps instead of 7.0 in Quake. Rejoice!
I'll talk about battery life, stability and amount of smoke produced after I use it a bit more.
Looks like I got a 400 MHz XScale PPC for only $200. Not bad, not bad at all...
For some reason, it took more work than it should've. In case you need any instructions as to how to do it (results may vary, and there's probably a better way to do it since I did this via trial and error):
1. Get Speed Stepper and run it
2. Check the box that says "show more frequencies" and select 398 MHz in the box on the right
3. Axim should lock up, reset it
4. Go to Speed Step again, and select 299 MHz, again in the box on the right. Your Axim shouldn't lock up and you should be able to quit the program.
5. Go into Speed step again and your current speed will be 398! Run Quake or any other games to see the difference.
Sadly, turning off your Axim will reset it back to 300. However, after turning off or doing a soft reset, you only need to do step 4 to go back to 398.
And yes, this probably will kill your Axim. Use at your own risk.
Oh, and the Axim does get quite warm compared to at 300 MHz. Does the 400 MHz Axim warm up noticeably too, or is this a problem?
