If you are developing the app for MIDP2.0 devices then better a POST request using https connection will do.
And further using http/https doesen't matter as long as you are not sending the password as plain text to the server. The one way to sent have authentication mechanism is to sign the password in the client end and send only the password signature to the server along with the username as plain text.
Hope this answers your query.
~Mohan