Every time I start gparted it greets me with a window telling me that "The kernel is unable to re-read the partition table on ..." and also tells me that I need to unmount all partitions on the device. 

This is nonsense because the drive contains the root filesystem, which will be mounted in nearly all cases (livecd excluded). Also, this has never been a problem in the past?

The console output is actually a bit more informative:

======================
libparted : 1.9.0
======================
The kernel was unable to re-read the partition table on /dev/sda (Device or resource busy).  This means Linux won't know anything about the modifications you made until you reboot.  You should reboot your computer before doing anything with /dev/sda.

Well, rebooting will of course mount some partitions on sda again.

Either don't show this messages or make it that gparted cannot be used on the drive it runs from.