I ripped this (my own DVD) with disk utility, so it's safe, for those of you who might be concerned. Also it does not need a serial number. Apple doesn't require those for any of it's core products. They assume you'll buy them and they don't need to lock you down. ;-) But seriously though, if you like this you should consider buying it (not to get too preachy or anything).

You do not need to burn the .dmg file in order to install. Only do that if you want to back it up or install on a computer other than the one on which you downloaded it.
MD5 : cbff081ed8d154c2f274f6bdb7863917