user=<username>&password=<password>&version=<launcher version>

with a "application/x-www-form-urlencoded" Content-Type header. The current launcher version is 12, sending a value lower than this will cause the server to return "Old Version" however you can send any large number and it will return as expected. If the login succeeded, it will return 4 ':' delimited values.
1281688214000:a348b469d915a40c9eebd2b9919c1a39:TkTech:8204407531530365141:

The first value is the current version of the game files (not the launcher itself). The second value is your download ticket and used to be required to fetch new versions of minecraft.jar from the server. The third value is your case-correct username. The fourth value is your session id and is required to connect to servers.
 The New Launcher

The new launcher uses the same basic protocol as the old one, but at a different ho