I realized that if Playmaker custom actions have any sort of compile error in them, Playmaker will not load correctly in Unity on Unity startup. This is probably a bug on Playmaker's part, and it'd be cool if it got addressed. =D
To repro this:
Create a custom Action.
Leave a compile error in it.
Save project and Close unity.
Try to open the project back up.
Notice no menu bar item for Playmaker, and no access to Playmaker GUI.
The work around for this is to comment out (or fix) the compile error issues, save the script, then relaunch Unity.