This is just a note for anybody that would think of doing this (as the xDSL
lines as so affordable now)
It can not be done with just BM due to a single default gateway.

I spent weeks testing it and here are the results:

Xincom XC-DPG502 claims to be able to do it, but it is a piece of junk. Yes,
it can do it (sort of) in a crude flimsy way. And I would say 15
simultaneous users is the maximum

There are other balancers that look exactly the same if looking at the
configuration pages(Firebrick, Agile Netwalker, Neteyes, Edimax PermaLink
PRI-682 and few others)
I can not comment on them, as I have not tried them.

But Xincom is just rubbish!!! with NONEXISTING support

The alternative is an expensive Xrio S400Lite

This unit is just magic. Lowest spec from the range, can balance 3 xDSL

And it just does it. Support is phenomenal and the unit is just a
Setup is a breeze (once one understands that all is virtual, as each port is
marked LAN physically)

The unit has almost endless configuration possibility. I could not find
anything that I wanted that could not be done.

Balances the lot out beautifully, but fits completely transparent. So if one
still wants to run internal services (that must be accessible from Internet)
ie. VPN,FTP etc, it is not a problem

And higher models can even load balance the inbound connections
Unit is a small box, running Via C3 400 based motherboard, with 64Mb CF &
64Mb memory
Never gets hot

The OS is a Mandrake (I think) Linux, with few custom applications

Anybody thinking of load balancing few xDSL lines on BM server should give
this unit a try

Expensive, but you get great piece of hardware/software solution