Hi! Horse's mouth here, as someone on this forum once referenced me. Yes, the LPC900 devices have 5 V tolerant I/O, except with some special cases, which I will explain. We have said this in our literature, in our user manuals, but actually haven't had an actual "d.c. characteristics" spec that says you can connect 5V to an I/O pin until last week when we started to add that to the datasheet. It'll take a little bit of time before these revised datasheets appear on the web.

The two xtal pins are not 5V tolerant. You can go up to 4V on these. Any pin with an analog alternate function is 5V tolerant provided that the analog function is disconnected from that pin or disabled. For example if you have selected CIN2A and the internal Vref as the inputs for comp 2, then the I/O pin that has CIN2B on it is 5V tolerant. On the A/D inputs, they will be 5V tolerant as long as you don't select that input in the ADINS register or if the A/D is disabled.