I have a ThinkPad 420 (Intel HD Graphics) connected to a Dell 3011 monitor through DisplayPort, and I started having the same problem after upgrading from Ubuntu 12.04 to 12.10. I get about a minute of usage before the monitor goes blank, unplugging the DisplayPort and plugging it back in gives me another minute of picture. With 12.04 I had no problems, and used it with this monitor for about a month.
The only "fix" I've found so far is to boot into one of my old Ubuntu 12.04 kernels (I'm typing this on 3.2.0-32). This improves the situation a bit, but the monitor still periodically goes into power save mode (sometimes after 30 minutes, sometimes I get a few hours). 3.2.0-32 seems to work just fine with the 12.10 user space.
To boot into an old kernel, just hold down the shift key while the system is booting (while your BIOS screen clears) and choose Advanced from the GRUB menu. Unless you uninstalled your old kernels after you upgraded, you'll probably have a few 3.2.0 kernels still in the list.
If you didn't upgrade to 12.10 (fresh install), I don't know an easy way to install kernels from an older distribution, but there's probably a trick to do it with apt.
I'd file a bug if I had any idea what to file it against. I originally suspected this was an X issue, but the old kernel certainly changes the behavior, and I had no problems with 12.04. May it's a DRM thing, or a DRM + X thing?