When Sony released the PSP firmware 3.50 end of May, they did not disclose that they increased the CPU clock speed to 333MHz from 266MHz for upcoming UMD games.

Current available games will not benefit from the new CPU speed of the Sony PSP. Apparently there are firmware hacks that enable to run games at full CPU speeds.
Sony most likely had the clock speed limited to extend the Sony PSP battery life.

There is no official statement from Sony, but ShackNews has confirmation from Sony that the full CPU speed is now available for game developers.
More details on ShackNews.