So, this is my problem:
I was able to install Ubuntu 11.10 in dual boot in my MacBook Pro 5.1 with rEFIt. I had to do it twice because first time I installed GRUB in /sda instad of in /sda3. Anyways, it wouldn't work: after choosing Ubuntu in rEFIt I got the balck screen with the white cursor.
Someone told me to try with an earlier version, so I installed Ubuntu 10.10. It works fine, but when I update to 11.04 I get the same problem: it won't boot Ubuntu anymore.
Any idea of what can I do to update Ubuntu? It looks like a problem with GRUB but I don't know how to fix it.