Take a look at http://kb.vmware.com/kb/1005241 for some options. Alternatively you can use vmkfstools to clone the RDM to a virtual disk.
Caution: Whichever option you choose, ensure the virtual disk's .vmdk descriptor file contains the correct size. If the RDM has been resized without recreating the RDM, the descriptor may contain the original RDM size and converting the RDM may not migrate all data (i.e. only the size shown in the .vmdk descriptor file!)
André
