Claro que esta puesto aposta. No he entendido mucho que quieres decir con eso de "si Linux le dijera a GRUB...", pero el caso es que funciona. Y lo hice el otro día con el PC de un compañero de curro en un Ubuntu 9, asi que casualmente lo llevo fresco. Te copio y pego el modo de hacerlo sacado del primer googleo que he encontrado apresuradamente:
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I am running Ubuntu 9.10 on an old Pentium 3 500 MHz, 500 MB machine. It runs ok as long as only 1 window is open. Nonetheless, here is how the sequence worked for my machine:

1) while the system is booting up, wait until the bios etc. completes - BUT GET READY! (you may have to do this twice since the GRUB message about ESC lasts ~ 1/2 sec.)

2) at the GRUB message HIT 'ESC' IMMEDIATELY

3) Follow the above sequence up to and including writing the word 'single' at the end of the command line that appears.

4) hit 'ESC' to return to the _previous_ screen. That is the screen where you selected to edit the kernel. Now type 'b'.

5) a new screen appears. select "Drop to the root command line."

6) you should now have a single-user prompt where you can type 'passwd'. That's it.

Reboot into normal mode and now you have root access to your machine.

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Creo que no hace falta decir, que este texto no es mio, es de un usuario del primer foro que he encontrado donde mencionaban el single mode en Ubuntu 9.