I used to have this on my two T61p's all the time and have asked the list previously. Switching to a text console and back sorted it usually, so I wasn't bothered too much. Using gdm, there was some kind of a trick I had to do to get the mouse working - I had move the mouse cursor around before the gdm login window appeared; If I left this for later, it couldn't come at all and I had to restart gdm. Go figure...
Now unfortunately this is no longer a problem for me as both my T61p's packed (the well known NVidia substrate problem of the time struck them eventually, interestingly with a six month respite on one of them). My present laptop, an HP Envy 17, so far hasn't been able to run any Xorg server under NetBSD, neither the Intel 530 nor the GeForce 950M are usable for now; I run NetBSD only in text mode, plus a bunch of other systems with a gui (FreeBSD works fine, as well as each and every Linux I've bothered to try).
But I digress.