mounting a dd'ed device

[dninja at gmail.com (Robin Wood)]

Hi
If in linux I use dd to create a file from a whole device, say
  dd if=/dev/sdb of=sd_card.dump

then I want to mount a partition that is on that dump, how can I do it?

I know that if I dd'ed a single partition I could mount it as loopback
but having the whole disk as a file I don't know how to reference a
single partition in that file.

Robin