From the symptoms, and having Intel video, it sounds like you're hitting bug 966744:
"[i965] Resume from suspend leaves me with black screen..."
Update: a fix has been found! :-)
It is in Quantal and working for. For 12.04, it's in precise-proposed, as of now, 2012/10/10. All reports from there are good, too.
Upstream Intel developers are working with Ubuntu devs and users, there. You might be able to help by getting an xtrace. There are instructions in a bug comment here, but full disclosure, I couldn't get them to work. Others did.
Here's an update, from the ubuntu-devel list (the end of this email).
"For 966744, it seems a non-trivial mesa bug. Timo reported it upstream. Already present in precise, but doesn't seem to trigger as often as in quantal. Will need possibily some help from the unity team on that one to workaround it."
Here are the upstream X bug, and kernel bug reports. (Thanks to Launchpad's excellent bug linking.)