I make it in advance all the time and it never gets mushy. Who here hasn't eaten a bowl of it cold for breakfast, even it you warm it up it doesn't get mushy. I would be more worried about the pasta soaking up the liquid and making it dry.

I agree - you can undercook the macaroni just by a minute or two... and that way you can hold the casserole in the fridge until ready to bake.

I've made it in advance a ton and it's never been too mushy. I always under cook the mac.

I agree with DonnytG on this. Just undercook the mac. Look at it this way, Stauffer's has made fortunes selling frozen mac and cheese.

I can't see it being a problem. For the pasta to start to get mushy it seems that it would need heat. A couple of days or even several hours might be a problem. But an hour or so shouldn't be problem. Are we talking casserole? I would do it because it's all going to be mushy anyway. I have never had a mac and cheese caserole that was the perfect "al dente". Its a casserole: )

How far in advance? I think you could prepare the cheese mixture ahead and refrigerate it, but I wouldn't mix it all together or I think it would turn to mush.