This three line patch adds support for middle mouse button on Synaptics
touchpad (I have one of those on my HP Omnibook 6000).

This particular model is identified by driver as:

Synaptics Touchpad, model: 1
 Firmware: 5.4
 180 degree mounted touchpad
 Sensor: 27
 new absolute packet format
 Touchpad has extended capability bits
 -> multifinger detection
 -> palm detection
 -> pass-through port
input: SynPS/2 Synaptics TouchPad on isa0060/serio1
serio: Synaptics pass-through port at isa0060/serio1/input0
input: PS/2 Generic Mouse on synaptics-pt/serio0

It has two buttons below touchpad and three above. Following packets are
received by kernel from device:

left down	packets: 81 00 00 c1 00 00
middle down	packets: 80 00 00 c1 00 00
right down	packets: 82 00 00 c2 00 00
up 		packets: 80 00 00 c0 00 00

diff -urw linux-2.6.5-vanilla/drivers/input/mouse/synaptics.c linux-2.6/drivers/input/mouse/synaptics.c
--- linux-2.6.5-vanilla/drivers/input/mouse/synaptics.c	2004-04-10 13:21:10.000000000 +0200
+++ linux-2.6/drivers/input/mouse/synaptics.c	2004-04-10 13:10:49.000000000 +0200
@@ -470,6 +470,7 @@
 			 ((buf[3] & 0x04) >> 2));
 
 		hw->left  = (buf[0] & 0x01) ? 1 : 0;
+		hw->middle = (!(buf[0] & 0x03) && buf[3] & 0x01) ? 1 : 0;
 		hw->right = (buf[0] & 0x02) ? 1 : 0;
 		if (SYN_CAP_EXTENDED(priv->capabilities) &&
 		    (SYN_CAP_FOUR_BUTTON(priv->capabilities))) {
@@ -571,6 +572,7 @@
 	input_report_key(dev, BTN_TOOL_TRIPLETAP, num_fingers == 3);
 
 	input_report_key(dev, BTN_LEFT,    hw.left);
+	input_report_key(dev, BTN_MIDDLE,  hw.middle);
 	input_report_key(dev, BTN_RIGHT,   hw.right);
 	input_report_key(dev, BTN_FORWARD, hw.up);
 	input_report_key(dev, BTN_BACK,    hw.down);
diff -urw linux-2.6.5-vanilla/drivers/input/mouse/synaptics.h linux-2.6/drivers/input/mouse/synaptics.h
--- linux-2.6.5-vanilla/drivers/input/mouse/synaptics.h	2004-04-10 13:21:10.000000000 +0200
+++ linux-2.6/drivers/input/mouse/synaptics.h	2004-04-10 12:57:38.000000000 +0200
@@ -87,6 +87,7 @@
 	int z;
 	int w;
 	int left;
+	int middle;
 	int right;
 	int up;
 	int down;