If you are trying to get NetBSD/sparc to run on QEMU, don't use NetBSD < 5.0.1 (see here) or QEMU < 0.15. I didn't have any luck with those versions. Newer versions seem to work fine, however, I got a bunch of DMA related warnings from the esp driver, with QEMU 1.1.1:

(esp0:0:0): select; 11 left in DMA buffer [intr 18, stat 87, step 4]

Ignoring this eventually leads to the following, and the emulated OS hangs:

esp0: unexpected status after select: [intr 10, stat 93, step 0]
(esp0:0:0): select; 11 left in DMA buffer [intr 18, stat 80, step 4]

Trying different QEMU versions, 0.15.1 seems to be pretty solid to run NetBSD/sparc 6.0 with.

For some reason I didn't get the keyboard to work with any QEMU version, effectively blocking me from starting the installation. Some people seem to suggest using QEMU's -k option, however, that didn't work for me, either.
In my case it looked more like as if the emulated NetBSD simply didn't detect any keyboard.

Turns out that switching to non-graphic mode (with QEMU's -nographic option) seems to be a valid workaround by sacrificing the graphical output. This uses the serial port of the emulated machine, so no keyboard device is needed. Works like a charm.

For reference, my setup is FreeBSD/amd64 9.0 as host, running QEMU 0.15.1, emulating NetBSD/sparc 6.0.