Is there an automated way to convert an rdm to a vmdk?
As per me, it should be possible. Virtual compatibility would be easier than physical compatibility .
Whatever we could do by using web client or VI client should be doable by using vSphere APIs. You may want to dig in : VIJAVA open source project: Its java based. Look into storage related APIs from vSphere API reference.
Here are clear cut steps for conversion:
Virtual Environment: Converting RDM disk to VMDK
Please take a look at VMware KB: Migrating virtual machines with Raw Device Mappings (RDMs) for options with migrations. If you are using a stand-alone host (i.e. no vCenter Server in place) the conversion can be done from the command line using the vmkfstools utility.
André