When I run a diff or merge, I would like the window to span all monitors so I can see the entirety of both files at once. Is there a windows 7 shortcut to do this?
Edit: Since the hotkey doesn't exist, how about something that is light and quick to setup that runs a configurable script to do this? Free is better.
There isn't. Due to the many different possible monitor configurations a feature like this is impossible to implement properly (thus, I claim that it wasn't implemented).
For example, look what happens when I maximize Notepad over all screens:
In the middle, on the main screen, the top and bottom are cut off because the outer screens are simply higher.
That being said, I actually used exactly the functionality you describe to make that screenshot. But I used the functionality provided by UtltraMon (commercial).
It has a maximize to desktop functionality: 
There are many utilities that add this functionality to Windows.
Before installing a third-party utility, check if your video-card software has built-in support for spanning over multiple monitors.
Otherwise here are some utilities :
VirtualScreenMaximizer (free), review by lifehacker available.
This SuperUser answer for How can you maximize a window on to dual monitors in Windows 7? contains an autoit script for maximizing over two monitors.
DisplayFusion, free version and pro version ($25). The free version might contain this feature.
Actual Multiple Monitors, free version and pro version ($29.95). It's unclear whether the free version contains this feature. Its "Maximize to Desktop" function can maximize to more than two monitors :
