The "cutting edge" version (1.71.5) published yesterday, was a bad release. Some users reported that it just didn't start after the update procedure. It looks like only MAC OS X Tiger users were affected by the problem.
The error has now been fixed, and a new update has been released a minute ago. Use the "Check for updates" feature to load it.
If your Mailplane doesn't start, you need to download the latest version manually: Please use the download page to either download the stable or "cutting edge" version.
I deeply apologize for any inconveniences this might have caused, and I will work hard to make sure I reduce the likelihood of this happening again.