You should keep in mind that Cisco routers have a limit on the number of ICMP messages they will originate per second. (My mind fails me as to whether it is 3 or 30). So you might lose a few ICMPs because of this.
David
Based on:
http://www.cisco.com/warp/public/707/GSR-unreachables-pub.shtml
"ip icmp rate-limit unreachable n
Where n is the number of milliseconds between two consecutive ICMP unreachable packets. The default value is 500. That means that one ICMP unreachable packet is send every 500 ms."
Also specified in:
"IP Fragmentation and PMTUD"
http://www.cisco.com/en/US/tech/tk827/tk369/technologies_white_paper09186a00800d6979.shtml
which was last updated about 10 days ago and covers everything in this thread so far (other than JunOS :-))
-Hank