Mac Latency Problem Solved
After talking to Apple for over 2 hours on the phone (most of that on hold trying to get to someone who could actually help me), I actually came away with a fix for the intermittent latency problem I've been having on my MacBook Pro since getting it.
The problem was that about every 6 seconds, the wireless connection would hang for about a second. This would show up in ping like this:
PING 192.150.23.1 (192.150.23.1): 56 data bytes
64 bytes from 192.150.23.1: icmp_seq=0 ttl=64 time=2.457 ms
64 bytes from 192.150.23.1: icmp_seq=1 ttl=64 time=1107.883 ms
64 bytes from 192.150.23.1: icmp_seq=2 ttl=64 time=109.092 ms
64 bytes from 192.150.23.1: icmp_seq=3 ttl=64 time=3.383 ms
64 bytes from 192.150.23.1: icmp_seq=4 ttl=64 time=2.060 ms
64 bytes from 192.150.23.1: icmp_seq=5 ttl=64 time=2.034 ms
64 bytes from 192.150.23.1: icmp_seq=6 ttl=64 time=1107.129 ms
64 bytes from 192.150.23.1: icmp_seq=7 ttl=64 time=108.167 ms
64 bytes from 192.150.23.1: icmp_seq=8 ttl=64 time=2.111 ms
64 bytes from 192.150.23.1 : icmp_seq=9 ttl=64 time=2.049 ms
64 bytes from 192.150.23.1: icmp_seq=10 ttl=64 time=2.161 ms
64 bytes from 192.150.23.1: icmp_seq=11 ttl=64 time= 1103.365 ms
64 bytes from 192.150.23.1: icmp_seq=12 ttl=64 time=104.465 ms
64 bytes from 192.150.23.1: icmp_seq=13 ttl=64 time=1.947 ms
64 bytes from 192.150.23.1: icmp_seq=14 ttl=64 time=2.157 ms
64 bytes from 192.150.23.1: icmp_seq=15 ttl=64 time=2.041 ms
The fix Apple suggested was deleting a number of configuration files, and that did fix it. I suspect the one that was causing the problem was:
/Library/Preferences/SystemConfiguration/com.apple.airport.preferences.plist
Deleting that and rebooting caused the latency to just go away; now I have a consistently fast wireless connection.
Diffing that file between what I had before and what I have now, I can't see anything that would obviously cause this problem, but some 802.1X settings are gone, and a number of settings like InterferenceRobustness and JoinMode that were previously specified are not in the new file. Maybe it was some interaction between the settings in this file that was causing it, I don't know.
In case anyone is having other wireless issues, the files that Apple suggested I move aside were:
NetworkInterfaces.plist
preferences.plist
com.apple.airport.preferences.plist
com.apple.network.identification.plist