Root Explorer and/or Super User File Manager (SUFBS) both have methods to mopunt system as read/write. But BEFORE you go mucking around in your system, make a nandroid backup FIRST!! Because there are some files that once removed, your phone could go wonky and/or some that don't like to be edited while the phone is running.
Now if you just want to use Terminal Emulator here's some commands (not the parts in paranthesis):
Code:
su (make sure you look for the 'allow superuser' pop-up on your screen the first time and select 'remember' as well)
mount -o remount,rw /dev/null /system (this will make system read/write)
mount -o remount,ro /dev/null /sytem (and this will set it back to read only)
exit (will drop you back to the "$" prompt from the root "#" prompt)
good luck, but be carefull . . . . . . .