I'm trying to use GParted from the Ubuntu live CD and it won't allow me to resize the main extended partition. There is a "key" symbol next to it and the "unmount" option is grayed out.
There probably is a secondary partition in that extended partition (like /dev/sda5 within /dev/sda2 in the picture)
EDIT: you did forget to follow my first sentence :-)
First you have to make sure that that partition is not mounted and then you must ensure that it leaves enough space inside the extended partition to make it smaller (if you want that).
If there's a swap partition inside of the partition that you want to resize, you'll have to "swapoff" the swap partition first. You should be able to resize the other partition afterwards.