The 2930 is not as good as the 2910. The 3910 is the successor to the 2910. Denon have always like Apple been very very secretive when it comes to releasing products. If the decide to replace the S301, then they will release a sheet containing that info. More than likely the S301 is the only machine Denon will make in this range worth getting. I have had mine for over 6 months and it produces a fantastic picture. Never tried it with an LCD or Plasma as I don't own one, but I doubt if when Denon do release a successor to this product, it will be a DVD unit. DVDs are still selling far more units than HD-DVD and BD, but DVD is not the future for a vast majority of customers, so it would be financial suicide to produce a top-end product that upscales to say 1080p, but it only works with DVD.
The 3910 was from the same range as the 2910, but more expensive. They have both been replaced (last October, I think) by the 3930 and 2930, respectively. The general impression I get from reading posts on here and from reviews is that they have been received as being superior to their predecessors, probably due to their different video chips, among other things.