Mailplane just got reviewed on Bright Hub! It's a great and very thorough review and describes what Mailplane is and what it isn't.
Here is what Bill writes about Security and Privacy:
"There are settings in each Gmail account's preference panel to control access to each individual Gmail account, by saving the Gmail account password to the Mac keychain and allowing automatic login as needed. [See Image 3] Of course, this is best used on a Mac that has password-protected user accounts. Mailplane also provides a setting in each Gmail account that enables https support. While this can affect the application's overall performance, turning on https is a good idea for anyone who's using Mailplane on a local area network."
Check out the full article, it is a great read!