From bug report: http://forum.piwik.org/index.php?showtopic=70
In the config file we should have
username = "my=username"
username = my=username
the same way the password is currently enclosed in double quotes
fixed in 
--> all non-array values in Config/config.ini.php are now automatically enclosed in double quotes, and double quote automatically encoded as " (parse_ini_file doesn't handle escaping the double quote, arg)
cleaning the Piwik_Config code, adding tests, the code using the Config is now more simple, all the logic is inside the class (before it was very hacky). the installer is now using the Config to create the config file rather than duplicating the logic.