I decided to upgrade my laptop to 10.04 beta 1.
When the update was done (something I had to do at home because the damn Update Manager does not work with proxies, at least not when running it manually) I was left without a working Xorg configuration, apparently because the fglrx (non-free) ATI driver did not support the x-server used in Lucid. According to the release notes, the fix is the use the open source driver (ati) instead.
After fiddling a bit, I discovered that removing
xorg-driver-fglrx and installing the package
fglrx did in fact work and I have accelerated OpenGL again.