As the IPv4 space wears thin — I have had to change my Bogon filters alot lately. (See this post.) Team Cymru is the best place I have found to keep up with the changes with out having to watch it daily.
Soon — we will have all of the usable IPv4 space allocated and I wonder how this will affect spam, IP Spoofing, and firewall and router loads. I think that these devices may have to work harder. Four years ago when I started dropping bogon sourced traffic at my network edge I had a 33% drop in spam and a 20% drop in abandoned connections.
The next question I need to ask myself is: how will bogon space apply to IPv6 (i don’t know the answer — but tell me if you do!) I will really have to look into an automated way to allow previously bogon traffic if it’s at all similar to IPv4