I have no idea why but sometimes a .dmg file will just-not-mount.
For instance I just downloaded the latest PostBox from http://postbox-inc.com/ and the silly little image will not cooperate.
My amazingly quick’nt dirty workaround? Convert it to another format file (using the command-line)!
hdiutil convert postbox-1.0b12-mac.dmg -format UDTO -o postbox_iso