After upgrading from Ubuntu 10.10 to Ubuntu 11.04, my MacBook Pro's touchpad stopped working.
It works fine on the login screen, but after logging in it stops working.
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(Answering my own question, in the hope that it saves someone else a bit of head scratching).
After the upgrade, your Gnome desktop configuration may have gone awry. There are a few settings which enable or disable the touchpad in certain contexts which may need tweaking to get the touchpad working again. Here's how I got mine working again: