I set my computers date to 2012-07-13 and tried to start the client (current master) and during startup it simply crashes. This is from the debug.log:

07/13/12 13:56:32 ERROR: CheckBlock() : block timestamp too far in the future
07/13/12 13:56:32 LoadBlockIndex() : *** found bad block at 34105, hash=0000000037c57b5ff0ebedd38469b9797c89e8640cbcd52dc967d3805e487b51
07/13/12 13:56:32 ERROR: LoadBlockIndex(): chain database corrupted

There should be a check and warning like the one in util.cpp - AddTimeData(), but this needs to be done from some core dev.