I found what seems to be a bug in the Mint 12 installer, at least in x64 anyway. because of lack of transparency it's a bit hard to say but this is what I did and what seemed to happen.
I wanted to re-prep my hard drive for a Windows and Mint 12 dual boot so I did the following:
Created a new partition table and partitions in Mint 12 live DVD.
Rebooted, installed Windows in the partition I made for it, went through the motions, checked everything was intact with mint live (or maybe it was my USB drive install. Unimportant).
Rebooted into live disc, chose other install option, told it what partitions I wanted Mint to use but didn't tell it to format because the partitions were blank and formatted already.
Because I told it not to format, the installer tried to back up pre-existing packages. Instead of progressing it just hung there. it didn't produce an error anywhere that I could find.
All I could do was reboot and try again. I did the same things except select format for the relevant partitions as it doesn't try to back up packages when format is chosen. It worked fine.
I can't say for sure but it would seem to be a problem in error checking regarding pre-existing directory structures.