Whenever I play Minecraft and try to walk and turn at the same time, I can't. When I hit keys on my keyboard, my mouse locks in place. (This happens both in and outside of Minecraft)
(Ubuntu 10.04 Lucid, Mac Mini Server, NVidia graphics)
There is an option called "Disable touchpad while typing". It might be mistakenly on. If it is for some reason set to be on, you can disable it by going to Mouse > Touchpad > and uncheck "disable touchpad while typing".
If that doesn't work it might be that your mouse and keyboard do not play nice with one another under ubuntu. Unless these are expensive peripherals, try to reproduce the problem with a different mouse or keyboard or both and see if that changes things.
synapse touchpad v7.2 OS: Windows Vista 64bit Laptop: HP DV7
search > mouse > device settings > settings > palm-check enhanced > palm check > off
had this issue with gaming. The directions above is what fixed it.