If you want to permanently
boot in text mode (runlevel 3) you have to edit the file /etc/inittab and change the following (and unique) line by substituting 5 with 3 (see bold number)
this is taken from Fedora 10, but it should not have changed in Fedora 12.
If you want to boot in text mode for one session only, you can pass the runlevel to the boot command line. Still in Fedora 10 (but should be similar in Fedora 12), when the grub menu appears, select the fedora you want to boot and in place of pressing Enter, press "a". You should see a line you can edit, e.g. something like
kernel /vmlinuz-2.6.x-x.xxx ro root=LABEL=/ acpi=on rhgb quiet
add a 3 at the end of the line:
kernel /vmlinuz-2.6.x-x.xxx ro root=LABEL=/ acpi=on rhgb quiet 3
and press Enter. You will boot in text mode only.