First thing that came up to my mind when i saw threads about overclocking was data corruption. And not so surprisingly Igor Stoppa (one of main engineers in n900 power management team) said that they will overclock but there might be data corruption due to the fact that OS is optimized to run on stock speeds.*

So I would say that we might already seen some "half" bricked devices :| People posting threads about camera not writing files or cases where app just stops working or hangs. Of course these might be real bugs but if the user experiencing problems has OC his/hers n900 then I would say to these people "talk to the hand" because there is no reason to waste time on devices that have been gone stress that they were not meant to be used. Or at least i would highly suspect any bug reports from people who have overclocked.

*
Just search his messages around here and in devel mail list.