This is such an annoying feature. I am always acidentally double clicking things. I don't understand how anyone could find it usefull It's pretty easy to disable. Just copy and paste to the mouse section of your xorg.conf somewhere under Driver "Synaptics"

Option "TapButton0" "0"

You could also do the same for TapButton(0-100), but I think Synaptic touchpads only have 1 tap button, which is the left mouse button, so TapButton0 covers it. It does for me.

I also throw in a:

Option "HorizEdgeScroll" "false"

because I hate when I accidentally navigate with it in Opera.

I have used this in Debian Lenny, ubuntu 9.1, and Opensuse 11.1. It's a Synaptics driver specific option, so it should work with any distribution that supports the driver, which is probably every one.