If your Virtual Hardware / Virtual BIOS is IDENTICAL then you can create / clone as many copies of your VM as you want since the activation is only needed ONCE.
While you can clone them, if you are running them on a network, you may run into issues since the GUID's and such are exactly the same on all of these clones as well. Therefore, it might be necessary to run something like sysprep to remove that uniqueness...which would then require activation.
MS is a bit cagey on this one -- in fact MS is a bit cagey on the whole idea of running Virtual Machines at all on normal Consumer desk / laptops.
Of course they are cagey, they want you to purchase licenses for each and every copy of XP you are running.
I can't really see this as being a problem since most people use VM's for testing - but if you were to say have 4 or 5 XP virtual machines running genuine Productive work CONCURRENTLY with the same key then that IMO is against the EULA.
Without a doubt this would be against the EULA. If you cannot run all of the software on 1 machine...then you need more licenses to run the software.
I think those who are IT types who would be most likely the ones doing testing requiring concurrent VM's would also have access to volume license software, MSDN software and TechNet software...which does provide the multiple licensing and such necessary.
I don't really see where an average home user, who purchased Windows 7 Professional, would "really" need more than 1 copy of XP Mode running to run their legacy software and hardware without new driver support.