GTA4 is as casual as Super Mario.  Games like GTA 3 SA cannot sell 15 million without selling mostly to casuals.  

 It is true, WiiFit also sells mostly to casuals and there is likely some overlap b/t the two groups of casual buyers.  However, they don't offer similar experinces and are both top in their class so consumers in that overlap area are still likely to get both.  Since both games are expected to have legs at least into Christmas, sharing a release date at this point in the year will not hurt sales for either game.

Id have to disagree when people say hardcore gamers wont buy wiifit, just look at the nintendo fans on vgchartz and see how many of those hardcore gamers are actually buying it.

Casuals tend to buy popular games, gta fits that profile, even if its classified as a hardcore game, a lot of casual gamers will buy it, maybe not the new casuals buying the wii's, but if u look at the ps2 generation of casual gamers there are plenty.

Game releases on other consoles are irrelevant in deciding on whether or not Wii Fit should be released.
BTW MK Wii releases 2 days before GTA 4 in Europe. 27 April 2008. MK Wii on its own is reason enough to purchase a Wii.

As many others have said, the timing of Wii Fit almost doesn't matter. Most games have to be very careful with the release window, as they do 50% of their sales in the first week or two. Wii Fit should sell for months (years?) on end, it doesn't matter if the debut is underwhelming. By releasing alongside Grand Theft Auto, Nintendo is trying to steal headlines away from the other consoles. Ordinarily I would say that's a losing move, but it may work out in this situation.

If anything GTA has more to worry than Wii Fit... GTA will be easier to pick up used than Wii Fit will. So if someone wants both, but doesn't want to get both at the same time, they will like take this into consideration and get Wii Fit first. Perhaps this is Nintendo's strategy to keep wii games at the top of the charts so people don't think about other consoles much.

I can see it being a dilemma for some people. A lot of fat women over the age of 40 are going have to decide whether they would rather pretend to beat hookers to death or pretend to exercise.

The trouble with using the word 'casual' is that it covers too many types of gamers both previous console owners and first time console owners.  FreeTalkLive is correct in that PS2 sales of the GTA series showed GTA series either cross a lot of hardcore genres (ie bringing in those that mostly play only FPS or RPG or Racing, etc gamers to all buy GTA),  but bought also in some fringes of the non-younger adult male crowd as well.  Which by their nature of buying only a few games are ranked as casual.

 Wii Fit casuals will be even further from those fringes.  Consider the moms of the gamers here that are interested in it.  How many also play GTA?  No, it will be more of the hardcore that crossover to also buy Wii Fit than the first time casuals who decided to buy GTA over Wii Fit.  Even the more 'experience' casuals will probably split evenly. 

And as already mentioned by jlauro, if you want both but can't afford both right now, buy Wii Fit.  Then in a month or so, buy a used GTA at a cheaper price.

Price and shortages is going to be a bigger problem than GTA4

Like has already been said, Wii Fit will have legs, but not a huge opening(compared to gta), and a completely different target market. Therefore, I don't think the games will likely affect each other much.

I want Wii-Fit, I want GTA4.

I'm going with GTA4 and then i'l get Wii-Fit at a later date.

I'm not too psyched about Wii-fit, it's not going to be as fun as GTA for me.