You will want to know C/H/S values for your destination media ahead of time with growimg. You will also need to know what hardware device name (sd0, wd0, etc...) your flash device will appear as on your destination system if you include the -rdroot option. The default device name in the flash image is wd0. You must set this variable correctly or your system will not boot.
Example using a 1GB CF with 1986 cyls, 16 heads, 63 sectors, with a disk target of sdX:
gzip -d flashimg.i386-20090904.gz
gzip -d szez.i386-20090904.gz
./cfgflashrd -image flashimg.i386-20090904 -rdroot szez.i386-20090904
./growimg -t sdX 1986 16 63 flashimg.i386-20090904
Note that the default flashimg file assumes VGA/PC console. If you want to reconfigure it 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.i386-20090904.gz
gzip -d szez.i386-20090904.gz
./cfgflashrd -image flashimg.i386-20090904 -rdroot szez.i386-20090904 -com0 38400
./growimg -t sdX 1986 16 63 flashimg.i386-20090904
gzip -d flashimg.i386-20090904.gz
dd if=flashimg.i386-20090904 of=/dev/sdXc bs=126b