I personally have not tried to activate more than one system with a single key cause honestly there is no need to. You get 10 keys for each type of OS listed (with the exception of servers) on technet so I used 1xWHS, 1xWinXP Pro 1x Win7 x64 Ultimate 1x Win7 x86 Pro. So I still have 9 more keys for XP Pro, Win7 x64 Ultimate and Win7 x86 Pro. If I could somehow use up all the keys I don't think I'd have space to walk around in my own house, let alone pay the cost for powering all those devices
Technet keys are retail keys but have been "flagged" to accept more activations because people on Technet are more inclined to change their hardware for testing since they are IT professionals or enthusiasts. Retail keys in stores don't have that feature because most people will install/upgrade an OS once then never change it again till the next OS release or they buy a new PC. At least that is the theory.