I've written before in the big Why not learn MySQL blog post, that comments in Stored Procedures in MySQL are useless.
The reason is simple - the backup/export/dump procedure does dump them into the file correctly, but when loading the file by piping it into the mysql command line client - the comments get stripped. This was a sort of an "optimization" in the client, which is now configurable.
The 5.1.23 change log says so:
mysql stripped comments from statements sent to the server. Now the
--skip-commentsoption can be used to control whether to retain or strip comments. The default is
--skip-comments. (Bug#11230, Bug#26215)
Too bad it's still not released yet, it's in development since September 2007 and the change log is already huge in comparison to the other 5.1.x releases.