As far as anyone can tell, the Pro-668 is the twin of the WS1080 (fraternal twin of the PSR-800). RadioShack is making claims on their website that it can do Phase II, so I would expect it to handle Phase II unless they wanted to resort to lying about their products. I don't know if you would be getting everything with the RadioShack product however. It has happened in the past that some accessory included with the Whistler/GRE products was considered an extra for the RadioShack product and required a separate purchase. It appears that they included a USB interface cable this time around, so I cannot think of anything that might be missing.
Firmware for Pro-668 is developed and delivered to RadioShack by Whistler. The updates for the Pro-668 would then be released by RadioShack at a time of their choosing to the end user. It could take any length of time for this to happen (it has taken upwards of a year before if I remember correctly). It is very important to note to never try to update your firmware with a version not specifically meant for your scanner. There are already plenty of posts that can be found floating around with people turning their scanner into a brick because they were under the impression that all scanners that look the same are the same.
As for being in stock, it seems that most RadioShack stores have them in stock according to their website (for my area at least).
