I am actually in the middle of doing some research on the same. I have lived within 3 hours of Yosemite for most of my life and never been there. I took my wife there a few weeks ago for a day trip and was amazed at the beauty! What an awesome place! We are going back the end of September (23rd anniversary) for 4 days at the Lodge at Yosemite Falls and I hope to sneak out early in the mornings to cast a fly into the Merced. I plan to do this early because the 1 thing that was terribly obvious was the number of people and absence of fishermen. From what I have gathered so far, the better fishing is away from the crowds (the no brainer obvious answer, but where the hell is that in a national park with 10,000 people in it) but then you've got to be able to get to it without any sort of expedition, so I may fish the obvious spots nearby at first light and just see how that pans out. I plan to contact some local shops and I'll post what I find out, you do the same. Best of luck to you!
Life is but a walking shadow. A poor player who struts and frets his hour upon the stage and then is heard no more. It is a tale told by an idiot. Full of sound and fury!! Signifying nothing. May as well go fishing!