All XBee shields I’ve seen have some kind of level shifter on-board, so I’m assuming that XBees are not 5V tolerant. However, the product manual (at least for the S2/ZB series) doesn’t seem to list any maximum voltages for the input pins, so I’m not 100% sure.
I suspect that this can be found out from the datasheet from the Freescale and Ember chips used in the S1 and S2 XBee boards, but I haven’t taken the time to look yet.