While we're at it, some more "benchmarks". On a MacBook Pro with a Crucial m4 SSD inside:

$ dd if=/dev/rdisk0 bs=1024k count=2048 2>/dev/null | pv > /dev/null
   2GiB 0:00:09 [ 215MiB/s]

Oddly enough, the blockdevice of the same disk had much worse performance:

$ ls -lgo /dev/{r,}disk0
brw-r-----  1    14,   0 Dec 14 08:13 /dev/disk0
crw-r-----  1    14,   0 Dec 14 08:13 /dev/rdisk0

$ dd if=/dev/disk0 bs=1024k count=2048 2>/dev/null | pv > /dev/null
   2GiB 0:01:06 [30.8MiB/s]

On the same machine, Debian/wheezy was running in a VirtualBox virtual machine, we're still getting half the performance:

vm$ dd if=/dev/sda bs=1024k count=2048 2>/dev/null | pv > /dev/null
   2GB 0:00:18 [ 109MB/s]

And inside this virtual machine, another Debian/wheezy installation was running as a Xen DomU virtual machine, performance halves again:

vm|domU$ dd if=/dev/xvda1 bs=1024k count=2048 2>/dev/null | pv > /dev/null
   2GB 0:00:29 [69.4MB/s]