I've not yet played with this version of Xubuntu (downloaded, however busy, busy, busy) but I have spent many a 'happy' hour playing with Xorg configurations a mice and touchpads so I might be able to help.  These things do seem to change from one Ubuntu release to the next (lucid was a nightmare), which why I prefer more stable distributions

I agree synclient is not the way to do this but it is a good way to find out what the Xserver thinks is configured (which should be what you observe by tapping but touchpads can be temperamental fellows).  From a console:

$ synclient -l | fgrep TapButton
    TapButton1 = 1
    TapButton2 = 2
    TapButton3 = 3

is the answer you want.  You won't get that with the configuration file shown in your posting.

I suggest, if you have not done this already, first you move your configuration to one side, restart the Xserver without it and double check with synclient the default setting is not what it should be.  Restarting the Xserver used to be Ctrl-Alt-Bsp but that was considered not user-friendly (seems a lot of users are really that cack-handed) and so it not the default any more.  If in doubt, just reboot.

Next is to find out if your configuration is being used at all.  Put it back in place, restart the Xserver and have a look in the Xserver's log file:

$ fgrep InputClass /var/log/Xorg.0.log
(**) Power Button: Applying InputClass "evdev keyboard catchall"
(**) Video Bus: Applying InputClass "evdev keyboard catchall"
(**) USB VoIP Device: Applying InputClass "evdev keyboard catchall"
(**) AT Translated Set 2 keyboard: Applying InputClass "evdev keyboard catchall"
(**) SynPS/2 Synaptics TouchPad: Applying InputClass "evdev touchpad catchall"
(**) SynPS/2 Synaptics TouchPad: Applying InputClass "touchpad catchall"
(**) SynPS/2 Synaptics TouchPad: Applying InputClass "Touchpad"
(**) SynPS/2 Synaptics TouchPad: Applying InputClass "touchpad catchall"
(**) SynPS/2 Synaptics TouchPad: Applying InputClass "Touchpad"
(**) Macintosh mouse button emulation: Applying InputClass "evdev pointer catchall"
(**) ACPI Virtual Keyboard Device: Applying InputClass "evdev keyboard catchall"

I see that no less than three 'InputClass'es are applied to my touchpad.  Two come from files in /usr/share/X11 while the third, named 'Touchpad' is mine and comes from a flle in /etc/X11/xorg.conf.d.

I notice that your configuration files has the same Identifier line as the original, so you won't be able to tell whether it is being used and you didn't change the file name so your configuration file has the same priority as the default and perhaps the Xserver is confused by this.

Try changing the Identified line:

    Identifier MyTouchPad

and rename your configuration to give it a higher priority (a larger numeric prefix):

    80-mytouchpad.conf

Now restart the Xserver and check the log again.  If your configuration is listed, check the TapButton settings with synclient.  If the tap buttons are the wrong way round still, change your configuration to read:

    Option "TapButton2" "3"
    Option "TapButton3" "3"

With any luck, all will be well.

Section "InputClass"
        Identifier "touchpad catchall"
        Driver "synaptics"
        Option "TapButton2" "3"
        Option "TapButton3" "2"
        MatchIsTouchpad "on"
# This option is recommend on all Linux systems using evdev, but cannot be
# enabled by default. See the following link for details:
# http://who-t.blogspot.com/2010/11/how-to-ignore-configuration-errors.html
      MatchDevicePath "/dev/input/event*"
EndSection

Section "InputClass"
        Identifier "touchpad ignore duplicates"
        MatchIsTouchpad "on"
        MatchOS "Linux"
        MatchDevicePath "/dev/input/mouse*"
        Option "Ignore" "on"
EndSection

with Option "TapButton2" "3" and next added by me.
After some reboots, this proved to have no effect, both with 2-3, 3-2 and 2-2, 3-3 correspondance in options.
The only way worked is using synclient to change its default mapping TapButton2=3, TapButton3=2 to TabButton2=2, TapButton3=3. Anyway, I don't recognize using synclient as a sound measure since its effect is ephemeral and doesn't last after reboot.
So, what I was doing wrong and how should I fix that touchpad bug? Why not having ability to configure taps with GUI tool?