No, Mailplane is not a subscription service. You pay for a Mailplane 3.x license. It's not time limited in any way and there are no recurring fees. All updates within a major version (say from 3.0 to 3.1) are included in the purchase price.
With that said, it is a paid upgrade to move from Mailplane 2 to Mailplane 3. Current Mailplane 2 license holders will be offered a discount of 20% on the upgrade to Mailplane 3. And if you bought Mailplane 2 on or after October 1 2012 you'll get the upgrade to Mailplane 3 for free. Learn more about the upgrade here.
Don't hesitate to write us if you have any further questions.