I don't know the MD3000, however I think your issue is comparable to a SAS connected HP-MSA. ESX sees the disk as a local disk and therefore does not allow to use the LUN as RDM.
The HP documentation http://h20195.www2.hp.com/v2/GetPDF.aspx/4AA2-2597ENW.pdf (pages 18 ff) explain how to use a SAS LUN as RDM, maybe it's the same as for the MD3000.
Thanks, I saw that link mentioned in a few places, but I ignored it because it looks complicated and they are not using ESXi and I didn't understand that the -z option they were using was a rdm passthrough/workaround (it looked like nothing more than a datastore at first glance). However, having exhausted all other avenues, I set about to try this. One, on ESXi it's easier to use the "hidden" console that the RCLI because you need to be able to see paths (maybe I just don't know how to do that on RCLI). Two, you apparently have to embed the vmdk file into an existing datastore directory
i'm a bit unclear on that one still. In the end, though, it appears to workit showed up on my Windows Server and formatted. We will see.
Thanks again for the link.
I just found http://www.vm-help.com/esx40i/SATA_RDMs.php which explains the steps better than the HP docu. In addition, it's based on ESX 4.0. What you basically do with the vmkfstools commands, is to create the mapping file for the RAW LUN.