Does this happen on other WMs as well? To me, this sounds like a driver issue. Some drivers (the binary nvidia one, namely) do have some options to control the aspect of the pointer. I never played with them so I don't know if this is one of the available options.
If you are using effects, try to disable them and see what happens. Also, did you update the driver (whatever it is) recently? -- Jesús Guerrero Botella
