There are obviously other dancing titles releasing, including Just Dance 2 and other dancing titles from Ubisoft, but the game is still selling over 100,000 units per month and should increase with Wii hardware in Nov / Dec and continue selling well into 2011.

The reason I ask is 5m is a hell of a lot.

On a single console, the only titles to top 5m this generation are:

Wii: Zelda, Mario & Sonic, WSR, WF , Wii Fit Plus, NSMB Wii, Brawl, Mario Party 8, Mario Kart Wii, Wii Play, Wii Sports, Galaxy 1

PS3: Modern Warfare 2, GTAIV, Modern Warfare

X360: the 2 Halos, the 2 Gears, the 3 CODs, and GTAIV.

Basically, if you aren't a Nintendo game, Halo, Call of Duty, or based on a kingpin from an earlier era (Mario & Sonic / GTAIV), you haven't topped 5m this generation, and so the fact that Just Dance is at nearly 5m is damn impressive, especially since its pretty clear (going by hatred from the male-oriented reviews on Metacritic) that the game is selling overwhelmingly to women. Women seem to really like it too - the game has bronze to gold rankings for every demographic of women on the Nintendo Channel.

Pretty impressive for a game that I know little or nothing about....outside that it is a dance game. As for 5 million, I have no idea, so your guess is a good as mine. 

You are missing Assassins Creed (70k to go and selling 4k/week) and Halo Reach for the 360 (Already 5 million most probably).

Missing Wii Fit Plus for the Wii (13 million and counting).

Missing MGS4 for the PS3 (80k to go and selling 10k/week)

OK, but still, Wii Fit Plus is a Nintendo game, MGS4 is a PS2 era kingpin title, and Halo is another Halo. It still doesn't really change what I said.

Assassin's Creed / Just Dance as the only completely new games not from Nintendo to top 5m on a single platform this generation speaks to how hard it is to establish a hit outside traditional genres.