I had a problem installing ML on a 2009 13"macbook pro.
After the restart the installation got stuck at the screen where it says "34 minutes to install" or something of that nature. During that period there is no visible progression. After a while, the installation stops with a message that a problem had occured (possible permissions problem, or hardware related issue) and that the HD should be checked/repaired in disk utility.
In disk utility the option to repair permissions was grayed out, even though it had indeed found problems. The repair function found no global errors.
After a restart the installation proces went exactly as before.
After running the hardware test the message was: 4HDD/11/40000004 SATA(0,0). So my line of thinking: hardware failure.
The next day at the Apple store the conclusion was: the HD can be saved but the system needs to be re-installed (read: the HD needs to be formatted). The information on my HD could not be saved. They weren't able to mount the HD properly. Apart from some tests they weren't able to get to the data.
To me, this sounds like a software issue which may be directly related to the installation app of ML. To be honest, I hadn't checked permissions before running the installation. I never had any issues, so it didn't even cross my mind. But if these permissions are indeed so important, I don't understand why a permissions check isn't an integral part of the upgrade proces.
It's still not clear what went wrong, but at this point the conclusion is: no data, but the HDD is OK.
If anyone has got some ideas about what went wrong, apart from using Time Machine/running a permissions check, I'm interested to hear them. For the rest of you, let this be another warning.