Autoconfiguration of Proxy settings on a temporary basis

My sales force spends about half their time on our network via a direct
connection or a VPN connection and the other half the can access the Internet
via their own ISP or a hotspot and such. They like this situation and I do
not want tto limit their Internet usage. While they are connected to the
company's network I want to force them to use our ISA 2004 server which is
setup as a cache only proxy server. I know how to force them to be one or
the other, but is there a way to force them to use our proxy server while it
is available, but allow them to browse the Internet when they are connected
to a network other then our own. I thought of a DHCP setting, but I couldn't
find one and I thought about a logon script, but I'm not versed well enough.
Any help would be appreciated.

Thanks,
MOOSJ