Okay, some additional information:
Even with my new Airport Extreme access point, today I saw the latency issue crop up again.
I had a look at my Airport settings in System Profiler, and noticed that unlike usual, the MacBook was using a 2.4Ghz channel.
64 bytes from 192.168.1.1: icmp_seq=1 ttl=64 time=36.006 ms
64 bytes from 192.168.1.1: icmp_seq=2 ttl=64 time=4.201 ms
64 bytes from 192.168.1.1: icmp_seq=3 ttl=64 time=21.760 ms
64 bytes from 192.168.1.1: icmp_seq=4 ttl=64 time=5.251 ms
64 bytes from 192.168.1.1: icmp_seq=5 ttl=64 time=16.989 ms
64 bytes from 192.168.1.1: icmp_seq=6 ttl=64 time=31.053 ms
64 bytes from 192.168.1.1: icmp_seq=7 ttl=64 time=3.157 ms
64 bytes from 192.168.1.1: icmp_seq=8 ttl=64 time=2.065 ms
64 bytes from 192.168.1.1: icmp_seq=9 ttl=64 time=4.249 ms
64 bytes from 192.168.1.1: icmp_seq=10 ttl=64 time=29.178 ms
64 bytes from 192.168.1.1: icmp_seq=11 ttl=64 time=238.861 ms
64 bytes from 192.168.1.1: icmp_seq=12 ttl=64 time=59.255 ms
64 bytes from 192.168.1.1: icmp_seq=13 ttl=64 time=67.123 ms
64 bytes from 192.168.1.1: icmp_seq=14 ttl=64 time=271.693 ms
64 bytes from 192.168.1.1: icmp_seq=15 ttl=64 time=1.238 ms
64 bytes from 192.168.1.1: icmp_seq=16 ttl=64 time=9.040 ms
64 bytes from 192.168.1.1: icmp_seq=17 ttl=64 time=39.751 ms
64 bytes from 192.168.1.1: icmp_seq=18 ttl=64 time=32.090 ms
64 bytes from 192.168.1.1: icmp_seq=19 ttl=64 time=52.312 ms
64 bytes from 192.168.1.1: icmp_seq=20 ttl=64 time=25.724 ms
Request timeout for icmp_seq 21
Request timeout for icmp_seq 22
64 bytes from 192.168.1.1: icmp_seq=23 ttl=64 time=64.573 ms
64 bytes from 192.168.1.1: icmp_seq=24 ttl=64 time=12.917 ms
64 bytes from 192.168.1.1: icmp_seq=25 ttl=64 time=133.862 ms
64 bytes from 192.168.1.1: icmp_seq=26 ttl=64 time=65.149 ms
Request timeout for icmp_seq 27
Request timeout for icmp_seq 28
So I reconfigured the Airport Extreme to have a separate SSID for 5Ghz, and connected to that explicitly using the MBP:
64 bytes from 192.168.1.1: icmp_seq=0 ttl=64 time=4.974 ms
64 bytes from 192.168.1.1: icmp_seq=1 ttl=64 time=4.790 ms
64 bytes from 192.168.1.1: icmp_seq=2 ttl=64 time=4.752 ms
64 bytes from 192.168.1.1: icmp_seq=3 ttl=64 time=5.579 ms
64 bytes from 192.168.1.1: icmp_seq=4 ttl=64 time=4.807 ms
64 bytes from 192.168.1.1: icmp_seq=5 ttl=64 time=5.760 ms
64 bytes from 192.168.1.1: icmp_seq=6 ttl=64 time=2.061 ms
64 bytes from 192.168.1.1: icmp_seq=7 ttl=64 time=6.779 ms
64 bytes from 192.168.1.1: icmp_seq=8 ttl=64 time=6.059 ms
64 bytes from 192.168.1.1: icmp_seq=9 ttl=64 time=6.163 ms
64 bytes from 192.168.1.1: icmp_seq=10 ttl=64 time=4.557 ms
64 bytes from 192.168.1.1: icmp_seq=11 ttl=64 time=4.568 ms
64 bytes from 192.168.1.1: icmp_seq=12 ttl=64 time=6.735 ms
64 bytes from 192.168.1.1: icmp_seq=13 ttl=64 time=5.833 ms
64 bytes from 192.168.1.1: icmp_seq=14 ttl=64 time=5.841 ms
64 bytes from 192.168.1.1: icmp_seq=15 ttl=64 time=4.438 ms
64 bytes from 192.168.1.1: icmp_seq=16 ttl=64 time=5.815 ms
64 bytes from 192.168.1.1: icmp_seq=17 ttl=64 time=4.733 ms
64 bytes from 192.168.1.1: icmp_seq=18 ttl=64 time=6.212 ms
64 bytes from 192.168.1.1: icmp_seq=19 ttl=64 time=4.612 ms
64 bytes from 192.168.1.1: icmp_seq=20 ttl=64 time=5.413 ms
In both cases I was busy playing a 1080p MKV over the air.
So there you have it.
It looks like the problem is that the new 2011 MBPs have problems when using anything except a 5Ghz 802.11n WiFi channel.
Apparently one of the areas 10.6.8 looks at is AirPort, I hope for the rest of you guy's sakes that this will be addressed then, as the solution should not be to buy a new router.
At least, for myself, I now have an easy way of testing whether they have really fixed it.
Everybody who has a 5Ghz capable router, are you able to confirm that this is the case for you too? e.g. force the MBP to use a 2.4ghz channel like 11, and then check what your latency is while using the network heavily, and compare it to 5Ghz numbers.