Hi,
   actually I've just measured some firefox heavy IO activity, but as Nick reported, it is a matter of seconds and in my case it doesn't make the whole system irresponsive as it was before the patch.

Luckily I had disktop running, so here is the output. I can say that the the slowdown that I felt was probably due to the large write of 5.5MB in 5 seconds you can see at 18:28:31 - some random IO activity I guess.

Sun May 11 18:28:11 2008 , Average: 657Kb/sec, Read:    2642Kb, Write:    647Kb

     UID      PID     PPID                       CMD   DEVICE    T        BYTES
    1000     5880        1                   firefox     sda3    R      2605104
    1000     5880        1                   firefox     sda3    W       646660
    1000     5875        1              transmission     sdb2    R        98304
    1000     5875        1              transmission     sdb2    W        16384
    1000     5807        1           multiload-apple     sda3    R         2049

Sun May 11 18:28:16 2008 , Average:1031Kb/sec, Read:    2965Kb, Write:   2194Kb

     UID      PID     PPID                       CMD   DEVICE    T        BYTES
    1000     5880        1                   firefox     sda3    R      2936832
    1000     5880        1                   firefox     sda3    W      2246792
    1000     5875        1              transmission     sdb2    R        98304
    1000     5807        1           multiload-apple     sda3    R         1366

Sun May 11 18:28:21 2008 , Average: 856Kb/sec, Read:    2842Kb, Write:   1438Kb

     UID      PID     PPID                       CMD   DEVICE    T        BYTES
    1000     5880        1                   firefox     sda3    R      2826240
    1000     5880        1                   firefox     sda3    W      1440504
    1000     5875        1              transmission     sdb2    R        81920
    1000     5875        1              transmission     sdb2    W        32768
    1000     5807        1           multiload-apple     sda3    R         2049

Sun May 11 18:28:26 2008 , Average: 607Kb/sec, Read:    2089Kb, Write:    948Kb

     UID      PID     PPID                       CMD   DEVICE    T        BYTES
    1000     5880        1                   firefox     sda3    R      2023424
    1000     5880        1                   firefox     sda3    W       971588
    1000     5875        1              transmission     sdb2    R       114688
    1000     5807        1           multiload-apple     sda3    R         1366
       0     5331        1                   preload     sda3    W          131

Sun May 11 18:28:31 2008 , Average:1144Kb/sec, Read:     254Kb, Write:   5468Kb

     UID      PID     PPID                       CMD   DEVICE    T        BYTES
    1000     5880        1                   firefox     sda3    W      5599737
    1000     5880        1                   firefox     sda3    R       131204
    1000     5875        1              transmission     sdb2    R       114688
    1000     6157     5970                    iostat     sda3    R        11652
    1000     5807        1           multiload-apple     sda3    R         2049
    1000     6157     5970                      bash     sda3    R          531

Sun May 11 18:28:36 2008 , Average:  19Kb/sec, Read:      81Kb, Write:     16Kb

     UID      PID     PPID                       CMD   DEVICE    T        BYTES
    1000     5875        1              transmission     sdb2    R        81920
    1000     5875        1              transmission     sdb2    W        16384
    1000     5807        1           multiload-apple     sda3    R         1366