trackpad moving but not clicking

When my macbook is awoken from standby the trackpad moves the cursor around as normal but will not respond to a tap or a click. A restart won't get it working again, but a little later it will start working once more. At the moment I can't identify what gets it going, possibly by clicking using the magic mouse but can't be certain of that.

Any ideas? Others seem to report erratic behaviour, but the problems seem slightly more mechanical due to oversensity of the trackpad or the trackpad not working at all.