This is not really a technical question, but an implementation
question (I hope that is still acceptable in this forum).

I want to put different offices on my OPT1 interface and keep the
different office separate from each other (ie. they can't see each
other).

My latest and greatest idea is to subnet OPT1 to /28 which will give
each subnet/office 16 IP's (with a total capacity of 16
offices/customers).  However for ease of use, I want each subnet to
have its own DHCP server and I don't believe m0n0 can offer DHCP for
each subnet (please correct me if I'm wrong).  I will assign each
office the first IP in their subnet to their Linksys WRT54G LAN.  Then
turn on DHCP in their Linksys and set it to start assigning IP's from
their second IP to their last IP for their subnet.  That way any
machine plugging into the Linksys router will automatically get an IP
within its subnet.

This idea will solve the problen of having to do a NAT behind a NAT. 
Each customer will only be behind my m0n0 NAT.  If worst comes to
worst and some of the companies require more than 16 IP's, they can
plug their LAN connection into the WAN port of another router and have
their own internal network (creating the undesireable NAT behind NAT,
but giving them more address capactiy).

I want all traffic to pass through the m0n0 for multiple reasons.  I
want to be able to shape traffic, so no one customer can use all the
bandwidth.  I also want to be in control of protecting my customers
(they are not tech savvy people and if I didnt protect them would be
full of viruses within the first week).  It will also allow me to
monitor all traffic flowing in and out for troubleshooting purposes.

Some of you are probably thinking "this guy is nuts" and I may be, but
this is the best idea I have been able to come up with so far.  I'm
still in the design phase and would love to hear better ideas from
others.  Thanks for your input!

Joe