FACT: (personal experience)
I found that when I added my PMD relocation cable, my cruise quit working. I suspected the PMD was starting to fail before moving it. (had a few instances of fishbiting, cold stalls, etc)
I ran for almost a year without cruise control, before my PMD finally failed.
Once the PMD did fail and was replaced, the cruise worked instantly. The cruise has worked without fail since replacing the PMD.
SPECULATION:
I can't prove it, but an observation I made was monitoring ODBII scan tool. One PID has the name duplicated (related to IP solenoid), It appears to be the solenoid feedback pulse width. It had a higher reading on the older PMD verse the newer PMD.
Without looking at the injection control code, its impossible to know for sure. It appears that the PCM monitors this signal and disables the cruise if it is out of range. But this out of range is not an emissions failure so it does not throw a SES code.
Have you seen any other symptoms of a failing PMD?