The Ultra 6 Plus was pretty much the same as the later first gen Pro systems hardware wise. The Ultra 6 Plus was the follow on to the Pro Pioneer. I agree with PapaBear that they might not be able to get the E2160 chip in sufficient quantities any more. This is the same chip that was used in the first gen Pro released in 2008.
Also one would think that many people looking at the Ultra 6 Plus would decide the Pro 6 is a better choice. For a little extra you get 2 years more warranty, a considerably faster CPU and a larger feature set.
The ReadyNAS 600 released back in 2004 still receives firmware updates from NetGear (4.1.9 is currently in public beta: http://www.readynas.com/forum/viewtopic.php?f=17&t=59222
). Even though the Ultra 6 Plus has been discontinued firmware updates for it are not going to stop anytime soon. New purchases of the Ultra 6 Plus (some units will be left in the supply chain) will still come with a 3 year warranty and even after that period one would expect firmware updates to still be made for the product and some help available on the forum here.
The Pro Business Edition and Pro Pioneer were released late 2008 and the Pro 6 and Ultra 6 Plus were released late 2010. Following on from that you'd possibly think perhaps late 2012 for the next gen but the Pro 6 is a really powerful NAS that can even transcode (convert on the fly) 1080p video. It's possible it could be in production for longer than that, maybe even an extra year. We'll have to wait and see what NetGear does. My advice would be to buy when you need it. As with all technology there's always something newer and better around the corner.