Hi,
I had a similar problem with my MBP - one day it would just stop at the grey screen whether attempting to boot from cd or hard disk. What I did was this - I booted it into target mode by holding down T, and connected it using firewire to another computer. This enabled me to get the data off and I was all ready to turn it off and take it to be repaired. However, since I had a linux boot disk spare, I booted the other computer from this, and plugged the MBP into it with firewire. I then used the lovely unix app dd and began wiping the hard disk of the mbp. I'm not sure of the exact command - there are lots of tutorials, and you just want to send "0" bits onto the MBP's hard drive.
In my case, I stopped this after a few minutes, and my MBP booted fine. The problem was a corrupted boot sector on the hard disk, and since this resides at the start of the drive, writing 0s to it sorted the problem.
So I recommend that you acquire another computer that you can connect by firewire to your MBP, which is booted in target mode. This other computer will need to be booted from either a linux boot disk, or be a mac, in order to read the HFS+file system. Gather all the data you need from it, and then you can wipe the disk on the macbook from whatever environment you're in on the other computer.
Hope this helps.