I heard from him this week. Apparently he has just been too busy with his day job right now to fix this right away. He says he will try to rebuild xournal using gnumeric's libart by the end of the month.

From my testing, gnumeric will use xournal's libart successfully but xournal will not use gnumeric's libart. So if I install xournal first and then gnumeric, gnumeric will have an error on installation but will install and seems to work. Not vice versa though. So as a temporary work-around, you can install xournal first and then gnumeric and things should both start.

/Mike