Kirby isn't out in Europe until '11 and I haven't seen Mario's 25th announced for NA so it's not as bad as that.
However Metroid: OM and Fling Smash are side scrollers as well but not likely to take many sales from Kirby/DK.
Really it's just a matter of if Kirby and DK will cannibalize each other's sales in NA (also add Sonic Colors in the mix) to which they probably will slightly but not tremendously as Nintendo fans and platforming fans will almost certainly get both. Casual players may get one or the other (or in Europe DK or Mario 25th) and then Wii Party (or Just Dance 2).
Personally I'm still debating if I'm getting DKCR. Never played a DKC game before (just the N64 game which was 'meh') but it looks like fun. Likely it'll get bounced in '11 for me as I'm still playing through Metroid and now have Kirby. Sooner get Goldeneye (if it's good), NBA Jam, or even Wii Party over another 2D platformer. For that matter, I still need to finish NSMBW.