adb shell
su
mount -o rw,remount -t yaffs2 /dev/block/mtdblock3 /system
This assumes your /system is yaffs2 and that it's at /dev/block/mtdblock3 the easier/better way to do this on most Android phones is:
adb shell
su
mount -o remount,rw /system
Done. This just says remount /system read-write, you don't have to specify filesystem or mount location.