Thanks for your help. It's almost working... First I was going crazy because the kernel didn't output anything, until a friend of mine reminded me I had to enable the serial console also in the kernel configuration. Now, the kernel is using the serial console but stops right before init goes live:
VFS: Mounted root (ext3 filesystem) readonly.
Freeing unused kernel memory: 240k freed
I noticed that if I use anything of SysRq, the output indeed goes to the serial console; but all other normal dmesg output like loading nvidia driver doesn't go to the serial console after init starts. Any ideas?