Although ISPs tend to let packets with RFC 1918 source addresses slip out from time to time, ...
maybe some isp's, or even most isp's in some parts of the world, but not isp's in general. we see a continuous barrage of rfc1918-sourced queries at f-root, along with a continuous blast of rfc1918-related updates in AS112. i don't think you want to use RFC 1918 as your poster child for getting filtering right.
... they're actually pretty good at rejecting RFC 1918 routes: currently, route-views.oregon-ix.net doesn't have the 10.0.0.0, 172.16.0.0 or 192.168.0.0 networks in its BGP table (there are two entries for 192.0.2.0, though). So in IPv4 the magic is of sufficiently quality.
route-views is run by competent people, and the networks who feed routing tables to it are usually run by competent people. filtering this kind of trash is probably a normal part of operations for this class of networks. i don't think you can use route-views as a poster child for filtering having been gotten right. -- Paul Vixie