Q v0.9.0d89 (and v0.9.0a89) running on Mac OS X 10.4.11. Guest is NetBSD 4.0, with no X11. If left running, the VSZ/RSS of process "i386-softmmu" continues to grow, as seen in the following:
bash$ while true; do vmmap 6616 | grep Mach | tail -1; sleep 10; done
Mach message [ 1464K]
Mach message [ 1628K]
Mach message [ 1792K]
Mach message [ 1956K]
Mach message [ 2116K]
Mach message [ 2280K]
Mach message [ 2444K]
Mach message [ 2608K]
Mach message [ 2772K]
Mach message [ 2936K]
Mach message [ 3100K]
Mach message [ 3264K]
Mach message [ 3428K]
Mach message [ 3592K]
Mach message [ 3756K]
Mach message [ 3920K]
Mach message [ 4084K]
This appears to amount to more than a gigabyte per day.
Drop me an email at stix@stix.id.au if you want any further info.