Actually, it does matter. You won't be able to activate a DM device on a partition that contains a mounted filesystem (2.6 kernels don't allow that).
To be more specific...
You can create the DM device, because device creation in DM doesn't necessarily imply that the device contains an active mapping. What you cannot do (on 2.6 kernels) is load a table into that device that uses a partition with a mounted filesystem.
Thus, the creation portion of your command worked, but the table-loading portion failed, which is why you see hdc5.000 in "dmsetup ls".