This is the result of a few hours worth of digging around, chasing after a loooong setup delay on SSH connections with (if I recall correctly) both the 2.3 SSH in 10.0.1 and the 2.5 SSH available from Scott Anguish.
If any of you have been experiencing long (dozens of seconds) waits in starting up SSH connections, it looks like 'arp' is being called with a parameter order that Apple's arp utility isn't handling as expected (Apple's utility wants 'arp -n -a' when SSH is using 'arp -a -n'). [Found this out by running ssh -v -v, after a bit of packet sniffing and DNS experimentation.]
Read the rest of this article if you'd like a workaround to speed up your SSH connections!
If you have references in your arp table that won't look up quickly (the -n is supposed to prevent symbolic lookup), that appears to be what imposes the delay. Whether these weird ARP cache entries are coming in through AirPort or System Prefs or a bug in some code, I know not.
Quick and Dirty solution is to flush out arp cache entries (which should be harmless) using 'sudo arp -d _____' for each numerical address in the 'arp -n -a' list that bogs down 'arp -a'.
Long term solution, I suspect, is to make the offending piece of code call the arp utility with the right parameter order, fix the arp utility itself to accept the different ordering, and/or find out where these entries are coming from.
I use the following Q&D script ('farp') to flush my ARP cache after waking up my machine
#!/bin/shYMMV.
BLEH=$(arp -n -a | cut -d '(' -f 2 | cut -d ')' -f 1)
echo $BLEH
for addr in $BLEH; do
sudo arp -d $addr
done
Mac OS X Hints
http://hints.macworld.com/article.php?story=20010424160627779