This is a new bug found in Ubuntu 10.10 using X server 1.9.0
Originally reported with nVidia drivers, there is an extensive thread finding similar problems on nouvea and ATI drivers as well.
Briefly, using Xinerama (either a single video card, or more commonly multiple cards) any or multiple applications kill the X-server, either instantly on displaying the app, or when the mouse cursor is moved into the screen area.
The full thread is on Ubuntu's launchpad page:
But some report the same problem with another Linux distribution (Xinerama).
Note, using the same hardware, but not Xinerama, works fine (within the limitations of not being able to transition between screens easily).
Backporting to earlier Xorg fixes the bug (except for some old instability in Xinerama).
The list of affected X applications is extensive, including Google Earth, kwrite, Opera, VirtualBox,...
I understand that Xinerama is not longer favored, but there is no alternative for many hardware configurations (Xrandr doesn't work with multiple display cards, or multiple GPUs on a single card like the nVidia NVS 420).
I'd be glad to test any configuration or report any more details, as would the rest of the bug reporters, I'm sure.
A patch has been submitted upstream.
Discussion from Arch Linux: https://bbs.archlinux.org/viewtopic.php?pid=841628#p841628
Fixed with commit 78f94f19aab66a1e5331df0ce29f36e310b4195d.