I'd heard that he was fleshing out both the "Ambassador" and the "vertical Warbird" for upcoming concept kits. Cool stuff.
I always thought Probert's version of the "Ambassador" was the more fun and interesting evolutionary bridge from the pre-Generations Excelsior
to the Enterprise
Of course, comparing Probert's design and Sternbach's design is somewhat unfair, since Sternbach's design was hindered by the "ship-of-the-week" restrictions on expense and ease of building a physical filming miniature. The image on Probert's web site shows that the two renderings of Probert's design (profile and perspective) don't quite match up, suggesting that it might have taken considerable refinement to make a working filming miniature from that design.
I was recently told that the saucer of Probert's Ambassador
is circular. I look forward to seeing this design in its entirety.