PM 4.5.3 beta 2 on a Macbook Pro...
If I hide PM using the X button, it cannot be "unhidden". The dock shows the triangle indicating PM is running, but clicking on it does nothing. Sometimes (not very often) right-clicking the PM icon and clicking Show works, but most of the time I have to quit PM and restart it. This behavior is different than any other application, and seems to have started with beta 2. Using option-H to hide works a little better, i.e. you can click the PM icon in the dock and it reappears, but even this isn't 100% reliable.