RE: On the back of other 'security' posts....

That depends on your definition of edge, I suppose. I define it as the
port on one of my routers where the other end of the link is connected
to a machine I don't control. In those terms, edge filtering makes sense
in some cases and not in others. If it's a dial-up or T1 customer which is
a single business, it makes sense. If it's an ISP with a few fortune 500
customers, it doesn't work out as well.

I agree. In the satellite world, such filtering is extremely difficult due
to the asymmetric nature of the traffic. A common scenario is that the
customer will receive packets from upstream via Provider A to addresses
assigned by Provider A. The customer will send packets upstream through
Provider B with source addresses belonging to Provider B. If Provider B
implements edge filtering, then the only way round is to use GRE tunnels,
which gets messy.