Will windows treat a restored backup as another use of it's key?

No.
The rearm count is the number of times that the system can be re-armed during a grace period (i.e. before it must be activated) not the number of systems it can be activated on.

Interestingly, installing SP1 increases the rearm count by 1 - I haven't checked to see what happens if you then uninstall SP1 (does the count decrement by 1?)

The only time the rearm count comes into play is if the system for some reason loses activation and goes into grace - you can rearm it to extend the grace period by another thirty days, while you sort out the problem.

You may have seen that many OEM installs have only 2 rearms left - this is because the OEM has used a couple in the process of sysprepping the system for distribution.

correct - it also breaks the terms of the Windows License to which you agree at the start of any Windows OS installation, and MS could sue you for damages.

You can still rearm 4 times. What's the problem ?

There is no problem - because rearming was not the original question.

Tomha said:

Oh, twould appear there has been a misunderstanding, I think, largely on my part as to how exactly windows activates. This was more an issue of how windows detects seperate systems, it would appear that it has detected 3 system changes when there have only been 2, so while I can rearm windows, if I were to change motherboard again, technically my license should allow me to install on 1 more "system" (to my understanding), but because of the way it has detected systems thus far, I would not be able to permanently activate the copy of windows using my license key. Does that make sense?

How did you restore win7 after swapping motherboards? Just backup/restore with a backup program? Or did you sysprep?

OEM license doesn't work after replacing motherboard. RETAIL license does work.

SYSPREP detects all new hardware after swapping motherboard. But it lowers the rearm count and asks you to reenter license key! Windows 7 Installation - Transfer to a New Computer

It would be normal anyhow, for a reactivation request, no matter what else, since changing the board would almost certainly mean changing the MAC address on the NIC, which of itself is often enough to trigger such a request.

Replacement of boards for failure with a manufacturer-approved 'similar' board is allowed under the rules.
Most of the time sysprep wouldn't make any difference to an activation request (in fact, as I understand it, sysprep would make such a request certain)