If it is, then what does it actually mean for the series? I mean Mario games are kinda of Mario games regardless of number. It's not like they haven't return to 2D gameplay before (New Super Mario Bros anyone?). Either way, it's bound to be a really tight and almost bug-free experience. Mario games never disappoint unless we're looking at sports spin-offs.
I've only recently fallen out of love with Dimps. I think they're great when they're doing their own thing and not trying to re-create the classics (eg Sonic 4). The Sonic Advance series was the best 2D Sonic series I could have asked for and the Rush games have been consistantly fun. Colours was a bit short on content, but it was still fun none-the-less and the colour powers went a long way to refresh the gameplay. Dimps shouldn't try and be Sonic Team (or be forced to be Sonic Team) because they're obviously not good at it. Generations could be great, but they really need to sort out classic Sonic's weight because he drops like a tonne of bricks.