Multiple LAN's through one vpn

[Sturla Holm Hansen <sturla@hitconsult.no>]

Hi there, I have a problem with the following scenario:
I have 5 customers in 1 building sharing 1 SHDSL-line, each of them have 
their own router behind the main router to segment the net and now they 
want VPN.
The problem is that they should not have access to each others LAN, just 
their own, so I have to figure out a way to determine what network they 
should have access to by checking what login they use on the VPN.
I know that multiple external IP's would solve the problem, but that's 
not an option.
So could I set up a Linux VPN-server that has different routes for 
different logins?
Any sollutions would be much appreciated :)

Sturla