Hammocks--made specifically for backpacking, not the kind you use in your backyard--have been popular among ultralight backpackers for a few years now. The kind I'm familiar with--Hennessy--has a sort of built in tarp so that you aren't unprotected from the elements, and it keeps you off the snow if that's an issue. But I don't know if you need sturdy trees (so, not the spindly subalpine trees you find in higher elevations), which would make them inappropriate for some trails. I think your better bet if you want advice about using these in Yosemite is to find a backpacking forum--ie at backpacking.com. I think there are also specific Sierra Nevada hiking forums. Try googling.

http://hennessyhammock.com/sp-ultralight.html

Here is the hammock if anyone's curious:
http://hennessyhammock.com/sp-ultralight.html

"<i>Hammocks--made specifically for backpacking, not the kind you use in your backyard--have been popular among ultralight backpackers for a few years now. </i>"

Yes - I know what backpacking hammocks are. I didn't think the OP was asking about sipping a margarita in a hammock between two palm trees. :)

But we are talking about high country Yosemite in the snow . . . .