Live storage motion is not supported as part of vmware 3.5. To move a disk to a different datastore, you must power down the server, move the disk by choosing migrate, and then power the server back up.
Live storage vmotion whilst the server is powered on is supported with vsphere 4.1 and above, which allows you to move a virtual disk between data stores whilst users are logged in. This may cause performance issues.
As always, ensure you have a backup incase issues occur.