I am not saying they are doing it, I am just saying it's to easy for them not to do it. In short words, a checksum is the following:
Say you have a file, the file is digitally stored and contains only 0 and 1, or if you go hexadecimal you have numbers from 0-F (16 different). 32bit, 64bit same ting just longer range (#000000-FFFFFF -> #000000000000 -> #FFFFFFFFFFFF and so on)
Short said: what a checksum does is add all the numbers in the file, and you get a final number - the checksum. One could easily look for all the network adapters, graphics card and such and do a checksum on theese aswell. Point is, when you install locally you give the software 100% access to your computer. Most computers are not configured the same, so they easily stand out from eachother.
Infact, but ignore this, they could install a backdoor which stores your Visa information. They could also scan all your files for sensitive information, and send it back to MG HQ. However - all software you install on your computer may do this... That is especially _why_ you should _never_ install unthrusted software, like you get on email from time to time. I would say MG is thrusted software, there should be no doubt about that. However when it comes to detecting a machine, I would not be surprised if they do add some extra information when you logon, and some checksums would be the logical choice, atleast that's what I would do (for starters, hehe).