I've managed to find an overclocker that works with ipaq 1930 and 1940 models:
http://sukin.newmail.ru/setclk.html
The site is in russian, as is the software, but it works (I can overclock my 200 MHz 1930 to 270 easily). The only issue for me is that I need to soft-reset the PPC to reset the CPU clock back to normal (though I don't see this as a serious issue)
Craig.