As far as i know there isn't an easy way to do this, but... I have a user who has laptop that I need to assign a static IP when he is plugged into my local windows-based network. When he's on the road (or at home) he needs to get DHCP.
Normally I'd just setup a reservation for the in-house connection to send him the same IP, but my issue is though (and I won't go into why) is that he needs a different subnet mask when in house than the one that the DHCP server is deploying. I need to change my subnet mask across the board, but that's a project for the (near) future and i need this going ASAP.