A Unity3D defect involving mouse handling is affecting most (but not all) players on Linux.
We really want to get this fixed, but we're at the mercy of Unity on this one. If you have a Unity3D community account, or know someone who does, you can help by spreading the word and lending votes to this issue:http://issuetracker.unity3d.com/issues/screen-dot-lockcursor-under-linux-breaks-input-dot-getaxis
The issue is that when locking the cursor during mouse look, the camera freaks out and the game can crash. We work around this by not locking the cursor on Linux, but that means when the cursor hits the screen or window edge, camera look stops responding.
It seems that Unity will not fix it unless enough users cry out, so all help in raising this issue with them is much appreciated.
In the next release (1.12), an option to toggle cursor lock will be available under Input Options. If you're one of the lucky few who this does not affect, you'll have the option to use mouse look normally. Please experiment with this option carefully.