Recently I get a lot of questions about sparc64 emulation in qemu. The only answer I can give, is the same one as "The Zombies" sang in 1960s: "She's not there".
But there is another Open Source (the project's page claims it is CDDL, in the sources I've seen GPL) project which targets emulating Sparcs. Actually, OpenSparc. So, if you are interested in the Solaris 10+ emulation, take a look in the Project Kenai's Legion Sparc Simulator.
If you already have a 64 bit Solaris machine, you can download a pre-built all-in-one (including the Solaris 10 image) package here.
The bad news are, there is no network card emulation, and currently build doesn't work under Linux. Should work under the x86 Solaris though, so it is not completely useless. Also it should be possible to port it to linux, since SunStudio is also available there.
But for now I'd be sticking to 32 bits and qemu.