gracie:~# time dd if=/dev/zero of=/dev/sdl bs=1024k count=10240
  10240+0 records in
  10240+0 records out
  10737418240 bytes (11 GB) copied, 344.745 seconds, 31.1 MB/s

  real    5m44.748s
  user    0m0.025s
  sys     0m10.152s

  gracie:~# time dd if=/dev/zero of=/dev/sdl bs=1024k count=10240
  10240+0 records in
  10240+0 records out
  10737418240 bytes (11 GB) copied, 217.106 seconds, 49.5 MB/s

  real    3m37.125s
  user    0m0.023s
  sys     0m10.268s

  gracie:~# time dd if=/dev/zero of=/dev/sdl bs=1024k   count=10240
  10240+0 records in
  10240+0 records out
  10737418240 bytes (11 GB) copied, 247.319 seconds, 43.4 MB/s

  real    4m7.323s
  user    0m0.036s
  sys     0m9.794s

EDIT: I ran it a third time just to make sure the results held. I'd still take this with a grain of salt, in my experience EBS i/o results are pretty inconsistent.