2010 NA and/or PAL releases:
Pokemon HeartGold/SoulSilver, Miles Edgeworth, Blood of Bahamut, Final Fantasy Gaiden, Infinite Space, Might & Magic: Clash of Heroes, Shin Megami Tensei: Strange Journey, Glory of Heracles, Inazuma Eleven, Dementium II (plus the ones which are not confirmed yet, but very likely, such as Dragon Quest IX , Professor Layton 3, Professor Layton 4, Luminous Arc 3)
2010 Japan releases (some of them are also possible NA/PAL releases):
Ni no Kuni/The Another World, Dragon Quest VI, Golden Sun DS, Professor Layton 5, Okamiden, Last Window, Estpolis: The Lands Cursed by the Gods, WarioWare D.I.Y.
The biggest hits will obviously be Pokemon HeartGold/SoulSilver, Prof. Layton 3/4 in NA/PAL, DQVI, Prof. Layton 5 and probably Ni no Kuni in Japan.
There will of course be hundreds more new releases as well and they will bring some sleeper hits which only very few (if any) people predicted prior to their releases, similar to successes such as Tomodachi Collection (Japan), Cooking Mama (NA/PAL), Transformers: Autobots/Decepticons (NA) or Puzzler Collection (PAL).
Back to the original question: Will 2010 be its weakest year in hardware sales? Yes, I think so, but is this a surprise with the sales it had until 2009?