As per the thread below I'm finding several people who's WHS systems are missing this file. They've all been running RC1 or better and have installed/uninstalled various Add-ins before encountering this problem, so it sounds like there's a rogue uninstaller somewhere or something more sinister is happening.
Either way it's a little scary that it's so easy for an Add-in to mess with the files required by WHS, is there anything that can be done to protect these files in the upcoming updates?
It appears that at the very least it is related to the WOL add-in. I uninstalled it today, and immediately got the missing interop.dll message. Not sure if there are others, but this one for sure deletes this file.