.htaccess password protect for a specific domain

[eluser]bretticus[/eluser]
Do...

www.mydomain.com
and
dev.mydomain.com

...point to the same directory and/or virtual host? If they do then you probably can't do this with an htaccess file (at least I know of no way to do a conditional that is a directive allowing host header matching.)

Instead, you could detect this in your application and generate the appropriate auth headers via code:

http://php.net/manual/en/features.http-auth.php