I'm seeing this bug on Interstellar Marines, but this is the first game I've encountered it on.
Ubuntu 14.04
Unity or Openbox - both are affected.
NVidia GTX670, 1920x1200, Proprietary Updates (331, I think) driver.

This is a horrible major blocker bug.
I'm using Kubuntu 12.04.4; both 3.8 and 3.11 kernels have this problem.
Window manager - kwin
on Samsung R580-JS02AU laptop with GT 330M and nVidia binary blob v331.-updates (not the v337 beta yet)

This is part of the stuff I posted to Muse Games for Guns of Icarus Online:

I turn my mouse and the sensitivity seems to be very bad [I later find out the sensitivity is 0.01 so I change it to 1.01 then 2 because it still isn't enough]. But when I turn it a lot (look around), it freezes and my Xorg process takes 100% of a thread. When I Alt-tab, GoI automatically disconnects that match (perhaps time-out for input) and I'm back at the main menu, all options enabled.

Each time I move the mouse around a bit, the game for some reason makes me look up in linear proportion to how much I looked around, in any direction. The problem seems exacerbated with lower mouse sensitivity in-game (GoI) because I must move my mouse around a lot more. There seems to be no issue looking around when the window has no focus (alt-tabbed to Steam window for example, I'm using kwin as part of Kubuntu), and when the mouse is hovering over the window, the player looks around just fine (fast and snappy according to mouse movement) without looking up bug and without slow look-around even at max sensitivity (2.0). I assume when I look around enough and the amount of "looking up" (which is increased in linear proportion to how much I looked around) exceeds a threshold, the Xorg process starts to max a thread 100% and the system gets really slow (my conky updates like a slow scan TV). I can get the game to come back and Xorg back to normal CPU levels by alt-tabbing then clicking back inside the GoI window. I do this multiple times over the log file.

I have attached the player logs. It looks like even when I get into a match I'll have that mouse issue. This is interesting because it looks like a similar bug to other Unity games like Paranautical Activiy, I'm stuck looking up (also in proportion to how much I look around, but it seems to be a lot worse in PA like the linear proportion constant is a lot higher). The mouse sensitivity seems a lot higher too, so perhaps it is also scaled by the linear proportion constant that the "look up" amount scales by. I can always look around a bit in PA (for like 0.1s if I roll the mouse quickly) but then the "look up" amount/bug kicks in after and I'm stuck looking up. In PA I seem to be looking up at the start without moving my mouse. in GoI, when I click (repair hammer bangs) the looking up _seems_ to stop when I click.
I play stock KSP with no problems, since it's not locked into my mouse view. (I need to right click drag to move the camera).

I noticed that the Unity 4.5 fix list includes "Linux: Fixed "jump" when initially locking mouse cursor."

Is that possibly a fix for this issue? I don't have a Linux machine to test this on, unfortunately, but it was reported by two players of my game a few months back and I'd love to do an updated build if the issue indeed has been addressed.

I know it still says, "Active" above, but I admit I'm unfamiliar with how up to date bug statuses are here.

Same problem on Ubuntu Gnome 14.04 with Gnome 3.10 driven by Nvidia GTX770M with the propietary driver, version 331.67.

I can confirm this issue with Surgeon Simulator purchased via Humble Bundle running on xubuntu 12.04 on two different systems, both using NVidia cards and multimonitoring setup.
Happens with Fullscreen / FullHD (or even 1600x1200) resolution, haven't seen this issue when running the default windowed 1280x1024 mode. Windows 1600x1200 also affected, so I suspect it has something to do with higher resolutions.
Note that the graphics output first stutters, stops, runs some frames again and then locks up. Sound and Game Logic keeps on running. Switching window focus (no matter if fullscreen or not) immediately fixes this issue, switching back will let you play some seconds, until the issue happens again.
Game is unplayable thanks to this.

One computer is running NVidia 304.116 with GF GTX 280, one computer newest NVidia driver available via steam repositories (sorry, not in reach yet) with GF GTX 750Ti. Both computer equipped with 4GB Ram and Intel quad core CPU.
Other (non Unity3D) 3D Applications and Games are running smoothly on these machines w/o any known issues.

He, we sold a few hundred copies of a Linux version of a game on STEAM, and there are more people wating in line. We are working in conjunction with NASA and really want to have Linux support, so please do this (if it's a quick fix, I will be happy, as I know the Unity folks are very busy).
Thanks so much!
Khal

*3.13.8-1-ARCH , Arch Linux x86 x64
*KDE , GNOME , It happens on both , happens less in windowed mode on some affected games
*Nvidia GTX660 , Nvidia driver , 334.21

This bug is ruining Unity games for me, and as someone who runs a gaming site it's essential basic things like mouse-look work correctly to video and review games.

I am using Manjaro 64bit with KDE, Nvidia 560ti with 331.49 drivers.

Same problem with Archlinux, Linux 3.13.4-1-ARCH, Mate (1.6.2) and Nvidia drivers 325.x to 331.49.
dont have the bug with XFCE (xfwm4) when starting in windowed

I've got the same problem with Archlinux, Linux 3.13.5-1-ARCH, Cinnamon (2.0.14) and Nvidia driver 331.38.