I'm pretty familiar with the GA-X58A-UD7, I have one and love it.
As you say, doing a quick check of all of the places I buy parts does indeed to indicate that they are not available any more. I'd guess that that will not change either.
Check out the GA-X58A-UD5 - it is very similar to the UD7 (if compare rev. 2 for both boards) and it's likely that if it has enough "slots" for your needs it would be equally compatible with BlackMagic's hardware.
Regarding what will supercede the UD7, my guess would be nothing really; probably their manufacturing plants are working overtime trying to get B3 (fixed) Sandy Bridge boards due to the Intel P67 SATA port fiasco. Then, they will be working later in the year to supply new boards for yet another chipset! The X58 has been around a LONG time.
I decided to go with the ASUS P6X58D Premium, which is a pretty nice board, well suited to overclocking, well designed from a heat standpoint, and supported by BlackMagic. I'll list the specs I came up with for my client in another thread for comments.