a potential workaround for this can be windows 'route' (cmd> route help for details),
the thing is you'd have to be using a service that has two different target IPs for the server
example:
Name: news.usenetserver.com
Addresses: 208.49.82.187
208.49.82.220
208.49.80.60
208.49.80.124
208.49.80.188
208.49.82.28
when I used to use this I had two valid gateways on my LAN, gateway1 (10.100.100.71) and gateway2 (10.100.100.51), (they where linked over a metro WLAN)
the cmds are something like this:
route add ip1 gateway1
route add ip2 gateway2
or for instance:
route add 208.49.82.220 10.100.100.71
route add 208.49.80.60 10.100.100.51
I was on XP SP2 when I was doing that and it seemed to forget the manual set route every few hours or at least once every two days, dunno specifically why, but to workaround that I created a scheduled task that would run a batch script containing those two lines ( plus an "@echo off" line at the top so it'd be silent)
same thing should work in windows Vista, and perhaps even win2k.
to remove a route use: route delete IP
to see current/active routes use: route print
Note, some NSPs don't allow connection over multiple IPs, ie: the Giganews Unmetered package only allows a user to connect from one IP at any given time.