I'm very happy now that I've managed to fix this very irritating problem. I have an Acer laptop rigged up to an external LG 17" monitor and every time I 'normal' boot, I see the usual Ubuntu logo and then just about when you would naturally expect to see the desktop - nothing. Nada.
I've been resorting to playing around in recovery mode, using xfix, removing all xorg.conf type files and rebuilding/rebooting numerous times to no avail. lspci revealed the ATI Express Radeon 200 adapter so that wasn't the fault.
I traced it back to the compiz app that was installed. I simply removed it with the following:
sudo aptitude remove compiz compiz-core
Now my desktop greets me every time I login, what bliss. Compiz should certainly be your first port of call if you have blank screen problems like I did.