But don't ask me why it is there.
I've seen it once in a while with my 10.10.
And with any 12.04 installation I did (beta and final), Grub was always shown at startup.
So, my words are not of help I guess.
Anyway - good luck.
@everybody - new problem occured:
when booting with the older kernel and no (!) propriet nvida driver, the system started up from standby into the same black screen as the shut down issue (with the little difference that the keyboard was working).
So going back to an older kernel and no nvida drivers doesn't solve it with my Dell Inspiron 1720.
Hmmm, I know that 12.04 is a just born baby, but ... - maybe I'm just some days patient with the developpers.