It is amusing though, that people are saying that the IPL is not the sole reason for this fatigue. Lets examine the facts.
1. India, by virtue of having signed off on the Future Tours Programme of the ICC are already committed to playing each of the other Test playing nations twice every 5 years (or possibly 4 years) - Home and Away. This agreement extends upto 2012.
2. The IPL came about after the FTP was in place, and therefore, commonsense dictates that it is the IPL which ought to be considered the additional burden on the players and not the schedule of the FTP.
Furthermore, the IPL as a tournament lasted for 5 weeks, which is slightly longer than your average 3 Test series lasts these days. Furthermore, the IPL almost certainly involves more travelling than a Test series does and hence is much more exhausting, even if the cricket is less strenous.
Besides, in these 5 weeks, all the players except Australia and Pakistan who completed a part of the FTP obligations with the ODI series in the Gulf and England and one visiting team (in this case West Indies) who would have played the scheduled May-June short Test Series in England (has been going on since 2000), would have been on a break. Including India.
So yeah, India would have had a break. The scheduling, if you take the IPL out of the equation, has infact been admirably standardized in international cricket in this decade. There is still some residual imbalance - England tend to play longer Test series than other sides, but even that has begun to be remedied in the second half of this decade. In terms of calender year, there is a naturally cyclical aspect to the FTP - in some years India tend to play 15-16 Tests, in others they play only 6-7.
To say that the IPL is not solely responsible for fatigue is inaccurate. It is, because it placed an additional extraneous burden on international cricketers. It is absolutely the outstanding reason for an unnecessarily cluttered schedule.
Besides, fatigue per se is not the issue. If you look at the amount of cricket English professionals used to play in the pre-war (and even post-war) summers (upto 35 first class games in a season - enough for a bowler to bowl a 1000 overs and take 100+ wickets!), followed by winter tours, you will see that they played enormous amounts of cricket - and players did well at the end of the summer. Im weary of arguments about tiredness, and arguments about scheduling - players who are tired have the option to opt out, as Dhoni did in Sri Lanka last year. Losing teams tend to make these more often than winning teams. In India's case, these are being made for them.
So did the IPL clutter the international schedule? Yes it did! Is that why India got eliminated from the T20 World Cup before the knockout stage? No.