It's a National Park, so carrying there will fall under the new regulations under discussion in
this thread, about the amendment to that effect that's stuck onto the credit card reform bill. The gist is that once these regulations go into effect (if they
ever do -- sigh...) possession and transportation of firearms in National Parks will be under whatever laws apply in the state where the park is located. So whatever you can legally do in CA as a whole, you'll get to do in YNP. And what you can't, you still won't be able to...
Until the new regs take effect, current National Park rules apply in Yosemite, which, I believe, are basically that you have to keep them locked up and unloaded...