It's nothing to do with the light sensor. Sounds more like Philips has sourced a different LCD screen. Old ones were Samsung. Anyone opened up a Pronto recently?

Remember the whole "reboot" issue was due to noise generated by the LCD. Is it possible this was caused by the new screen?

It's understandable that Philips is/was looking for new screen sources -- the whole reason there were/are so few Prontos available is due to a lack of LCDs.