I've added an About page.
The user Dark over at http://forum.dayz-ast.com/ created a bruteforce script which start at SteamID: 76561198300000000 , then hashes it with md5(the normal/right way) and then compares it with the guid put in, if wrong it subtracts steamid by 1 and tries that one. With bruteforce it takes an average of 1.5 hours for 1 guid, but it takes the same time for 150 guid's too.
Then I decided to make a simple rainbow table, here are the 3 first lines of the big file.
The file contains 470.000.000 SteamID's using 24,4GB
Instead of using cpu power by hashing steamid's to md5(guid) this searches a big file for the correct line.
The only bad thing about this design is doing multiple guid's, as i said above. Bruteforce takes the same amount of time depending of CPU power, but with rainbow table 150 guid takes 3 hours and 40 minutes
Sorry for long reply :)