To be more specific, I mean the file that keeps a history of every disk image/volume ever mounted. I know it exists because I found it once, but it was a long time ago and I have no idea how to find it again. It is similar to the InstallHistory.plist file, which logs every program ever installed.
You can find the information in
/var/log/system.log (or using the Console application). But the information is not kept forever.
Aug 4 07:34:34 ******** kernel: hfs: mounted External on device disk3 Aug 4 07:34:44 ******** kernel: hfs: mounted Time Machine on device disk5
and and a DMG
Aug 5 07:53:25 ******** kernel: hfs: mounted Xcode on device disk6s2
After a while
system.log is compressed and archived. To search in all the stored logs
cd /var/log/ (sudo cat system.log; sudo gzcat system.log.*) | grep mounted