Twenty20 is going to be the life engine thats going to be used to keep cricket alive pretty soon. Those who dont like T20 cricket, they are just not willing to accept that it has overtaken the other formats in INTEREST. It has already started a debate of whether the other forms will live or not. I can easily see a situation in the future where T20 games will be played like ODI's and test cricket will be played as it is (only between the top associate nations). One good thing that T20 will do is, minnows wont be so eager to jump into the test bandwagon. So we might see the quality of test cricket go up. Now they have T20 to make a name for their country in international cricket. So even if they dont get test status, they wont be dejected or whatever.
ODI's - I think ODI's might suffer big because its in between the two formats. ODI's were basically the entertainment package which differentiated it from tests. Now T20 offers more entertainment than ODI. And the duration of the format wont matter because even in ODI's, a minnow team can beat a top team if its their day (though chances of that happening are far lesser than it happening in a T20).