Something that has been bothering me lately about my ThinkPad (well, only since I installed Windows 7 on it) is that by default it disables the touchpad temporarily while you use the keyboard. This is usually a feature, to prevent users from accidentally bumping the touchpad while typing, but when I looked to disable it, I couldn't find any mention of such feature in the mouse/touchpad settings in Windows.
It turns out this is an issue caused by PalmCheck in the Synaptic settings:
The PalmCheck™ feature guards against operating the TouchPad with accidental contact. PalmCheck allows the TouchPad to recognize when your palm is resting on it or brushing its surface while you are typing. This will help to prevent unwanted pointer movement or clicks.
Although it doesn't mention disabling the touchpad while typing, it in fact does do this while set above 50%. To change the setting, open the Control Panel, then Mouse Properties, and select the UltraNav tab (other laptop brands probably use a different name for this). Click the Touchpad Settings button. You'll be able to find the PalmCheck settings under Smart Check Settings on the left.
I recommend setting it to around 50%, or have a play around to find what works best for you. You can test whether the touchpad is still being disabled easily by selecting a new value, then holding the Tab key and attempting to move the touchpad. If the touchpad stops responding after a couple of seconds, the PalmCheck setting is still too high.
I'm sure I'm not the only person who was bothered by this, so hopefully others find it useful.