No, the 5D Mark III is definitely not in a similar situation to the 50D. And as far as sales, not only judging from observation of those on this forum but also on the internet such as flikr etc, a great deal of people have already purchased a 5D Mark III and many more comment on their wishes for one or current intent of saving towards one.
Having been a 40D owner, I did decide to "upgrade" to the 50D instead of the 7D. While it was not a monumental jump, it was still a more capable tool to use than the 40D. Having used a 5D Mark II before the 5D Mark III, I can say and confirm that without a doubt the 5D Mark III is a noticeable upgrade from the Mark II, in various ways. The AF, the ergonomics/build, the frame rate, even the smaller things like the deeper bracketing for those of us who love to shoot HDR, etc. It is a great camera. I think what many people get caught up on is comparing the 5D Mark III as far as dollar value in comparison to an older, depreciated model. $2100 5D II vs $3,500 5D III. Any older last model will likely hold a significantly better price to performance value over a new model and this holds true for products throughout almost any industry. The 5D Mark II was $2,700 new. It would be fair if many were to think/ask, if the two were available brand new at their respective price points are the upgrades in the 5D Mark III worth $800? which would I buy?.. I, personally, would opt for the Mark III, which is really everything I wanted in the Mark II and wished I could have paid a bit more for to have.