libhildonwm/hd-wm.c is sending an invalid message when attempting to notify the system of a new application starting. This code path is only executed when attempting to start a service (i.e. when attempting to start a hildonized application that has set the 'X-Osso-Service' entry in the application's desktop file.
Any software component that is using the libstartup-notification library to listen for these startup messages will trip on a libstartup-notification bug that causes the process to segfault. To see this happen on a ubuntu-mobile build, edit the mousepad desktop entry and add:
"X-Osso-Service=org.moblin.mousepad"
... then start the mousepad application, and then attempt to start the app a second time using the drop down menu in the upper lefthand corner of the Marquee. Since matchbox-window-manager is using libstartup-notification, then the window manager crashes and odd behavior starts to happen as the system continues to limp along without a window manager.
I just pushed this patch into the ubuntu branch of the hildon-desktop bzr repository.