In kernel version 2.6.28, the kernel used in Ubuntu 9.04, the Graphics
Execution Manager (GEM) has migrated to the kernel. This should simplify and
improve the efficiency of X Server memory management, but requires some
changes to graphics drivers.

As a result old drivers will not work. This is a short term problem that
only affects those with kernel 2.6.28 and up and as new drivers are written
they will include this feature. That is not much good in your situation. If
you did an upgrade as opposed to fresh installation, try running the older
kernel which should still be in your grub list. If you did a fresh
installation search for a new driver from Nvidia or compile your own.