You can either grab an appropriate image and write it directly to disk:
gzip -d flashimg.i386.com0-20120831.gz
./growimg -t sdX flashimg.i386.com0-20120831
Or you can create a new image (default auto root, PC console), configure it, and write it to disk:
gzip -d flashimg.i386-20120831.gz
./cfgflashrd -image flashimg.i386-20120831
./growimg -t sdX flashimg.i386-20120831
If you want to reconfigure a PC console image for serial console, use the -com0 option to cfgflashrd. (Note that cfgflashrd will not revert the image back to PC console on subsequent runs.)
gzip -d flashimg.i386.wd0.pccons-20120831.gz
./cfgflashrd -image flashimg.i386.pccons-20120831 -com0 38400
./growimg -t sdX flashimg.i386.pccons-20120831
gzip -d flashimg.i386.com0-20120831.gz
dd if=flashimg.i386.com0-20120831 of=/dev/sdXc bs=126b
Uncompress the image and then use physdiskwrite