AFAIK (and I admit to not keeping up with the rumours too much) it is confirmed that the PS4 will not have a cell processor. This pretty much rules out backwards compat because emulating a cell on a non-cell would be a bitch, and Sony learnt from the PS3/Emotion Engine fiasco that putting a custom chip in your machines just for backwards compat is a bit of a mistake.
That pretty much leaves any PS3 DD owners with two outcomes - 1) some DD gets ported (probably the most successful) or 2) it just dies.
In the event that it gets ported, I'd imagine people being forced to double-dip if they wanted to play it on their new consoles. If it remains un-ported in the PS3 store then you can either continue purchasing PS3s to play your old DD content, or just write it off. Either way, you're significantly out of pocket.
This has certainly lead to my decision to not buy any DD software on consoles in the last year, and may play a large part in whether I go DD on any new consoles. Part of what I love about my hobby is my archive of stuff - my Saturn, my Dreamcast, my Snes, my GC etc etc, all with tangible software that I can collect. If software isn't tangible, I don't own it, I just lease it and my interest wanes somewhat.