I had this problem as well. I'm currently on Mountain Lion 10.9.2. No matter what i did i could not get the disk to eject in Disk Utility or delete the RAID set. I had created the 2-drive striped RAID set in 10.8, and at one point had encrypted the disk, then removed the encryption. Not sure any of that is related.
The log shows I got the following error:
2014-03-29 11:38:01 -0500: Deleting RAID âTMDataâ
2014-03-29 11:38:01 -0500:
2014-03-29 11:38:01 -0500: Unmounting volume for RAID set DC62E024-130A-4105-993E-D5A9ABB340EB
2014-03-29 11:38:01 -0500: Canât unmount RAID set DC62E024-130A-4105-993E-D5A9ABB340EB to delete it
2014-03-29 11:38:02 -0500: Error deleting RAID: Invalid attempt to unmount a whole disk containing a partition map.
The only solution I came up with was to physically remove one of the raid drives (they were in a firewire enclosure). I could then erase the other drive in Disk Utility. I then put the removed drive back in. Disk Utility showed the RAID set as offline and I could delete it.
After deleting it I recreated the RAID and am currently doing a secure erase writing 1 pass of zeros to the whole drive (just to make sure everything is read/writable).