Not really.
I think T20 cricket has definetely had an impact on the game's future. Close matches, stunning catches, brilliant hitting etc. But, T20 was brought into cricket to attract more people from other countries. There's no doubt T20 has caught the imagination of millions of people, but IMO a true cricket fan will always love the Test Matches and 50 over cricket games. T20 will be very successful, because it lasts for only 3 hours, and the pace of the game is always quick. There's always going to be some action going on.
Yes, the Test Match might be toooooo long, but still, it showcases the real stamina of players and we still have test match specialists like Rahul Dravid, Alistair Cook and Ashwell Prince to name a few.
The ODI matches are much more interesting, and they are my favourites and will be for at a couple years more. When ODI cricket was introduced, the interest level in it was stunningly high, and being the first cricket format to end in one day it was definetely eye catching. But, now, with the introduction of T20, people say there's no future for ODI cricket, but I honestly disagree. There are some amazing series coming up in Aus/India, Sri Lanka/England, Pak/SAfrica.
T20 may have a brighter future, but I don't think ODI cricket will fade away. If anything might happen interest in Test series may diminish IMO (not in rivalries like The Ashes or Indo-Pak though).