I'm not certain, but it sounds like a DHCP lease time issue. The DHCP server leases out the IP address for a certain amount of time (7 days is the default length of time). After half of that time has elaspes, the client issues a DHCPRenew request (Ibelieve that is the correct term, dhcprenew) to renew the lease.
Now as far as I remember, a powered off machine keeps its IP address info when you boot it back up. But, if the lease time has reached the half-way point or beyond, it would then request a new IP address from the first available DHCP server it could find.
What I think is happening is, your users are getting an IP lease, then moving to another site and plugging in BEFORE the IP lease has expired enough for them to need a new one.
So, JoeUser plugs in on Monday, gets an IP address from Site1 DHCP server.
On Tuesday, JoeUser goes to Site2 and plugs in. His IP address lease from Site1 is still valid, so it DOES NOT EVEN TRY TO GET A NEW LEASE! And IP conflicts occur, making JoeUser AngryJoeUser.
So, 2 solutions I can think of.
1) decrease the amount of lease time on your DHCP servers. Make it 2 days, so that way, every DHCP client will try to get a new IP address every day. It will add to the load your DHCP servers handle each morning, but then if JoeUser is bouncing around different sites, he will get his IP address and will be HappyJoeUser.
2) Teach your mobile users how to release/renew their IP address. Tell them about WINIPCFG. As I remember, it has a Release and Renew buttons. That will take care of them. They go to other sites, run winipcfg, and click the 2 buttons. They should be able to handle that.
Hope this helps.
