This has happened on my 07 Sport, I've only seen it happen in the summer though, still waiting to verify that through the next few seasons. The display gets scrambled but the player seems to still work. Sometimes I've had it spit out all the CD's in the changer. Each time this happens turning off the ignition and starting it up again fixes it.
Found someone here who's had the same problem:
http://townhall-talk.edmunds.com/direct/view/.ef13110/1733
Apparently from the service history of my 4Runner, a dealer did this under warranty: "REPLACE AUDIO UNIT WITH EXCHANGE UNIT" before I bought it. Seems that the problem still happens to the new unit.
Anyone else have this problem or find out the cause?