As you may already know the new version 3.5 firmware for the Sony PSP allows developers to use the full 333MHz of the PSP's processor.
However, DailyTech reports the 333MHz mode can only be used when the PSP isn't connected to the Internet:
According to more recent news from developers regarding the unlocked 333MHz operation, running at the highest clockspeed will disallow the usage of the handheld’s Wi-Fi feature, reports Beyond3D. The current developer tools only permit network play when running at 222MHz.
Although Sony has not revealed exactly why it has placed such limitations on developers – as the PSP is capable of running at 333MHz with Wi-Fi enabled – it is likely that the decision was made with consideration of the effect of either feature on battery life. Sony may have felt that enabling the higher clockspeed and wireless transmitter would result in inacceptable playtimes on a single battery charge.