The tests are perfectly good, it's how they monitor the tests which is wrong. They supposedly have an expert there to ensure that they are bowling as they would during a game. Now if my memory serves me correct, the expert at Murali's tests was the bloody sri lankan coach! Now i'm sure he was really subjective during the tests. But by the same token i'm sure it's bloody hard to bowl with all those ping pong balls over your body.
The thing that came out of the testing, is that it is IMPOSSIBLE to bowl a doosra without chucking - biomechanically impossiple to be scientific. They made that as clear as day, then there was talk that Murali would never bowl the ball again....months pass, he slips it back in the arsenal and the ICC somehow forgets the whole testing process. Pretty ludicrous huh? His offspinners are obvious throws as well (just watch him in brian laras cricket

, but he has gotten by through the "birth defect" arguement....which may very well be true, but that doesn't explain the other suspect actions in world cricket (like botha) nor his doosra.
Murali's record will always have a big * next to it, because several umpires, who are paid to make these decisions, called him for throwing. My opinion is that he bowls illegally, there's no good proof out there to say he doesn't, but there's hours of footage that suggests he does. But at the end of the day it doesn't worry a lot of the better batsmen, because he isn't that good a bowler. He bowls 80% of his sides overs so you'd expect him to take a few wickets, and has taken over 100 test wickets against crickets minnows in Zimbabwe and Bangladesh (I think Shane Warne has about 5) - genius? I think not.
Why hasn't Giles or say Symonds added the doosra to their arsenal? Apart from both being pretty ordinary tweakers, it's because they bowl with a straight arm, and it's impossible.