Touchpad problem?

You mean, the trackpad on the top of your MacBook, slightly below the keyboard (pictured below) is not working, when the MacBook (which exact model btw?*) is placed on a glass surface, though the trackpad is not even touching the glass surface?
Or do you mean a mouse (Magic Mouse) or the external trackpad (Magic Trackpad)?

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Identify your Mac and Mac OS X version, visualised in a more elaborate image
Identify your Mac and Mac OS X version using this 25 second video tutorial (3MB, .mp4, H.264)​

 

You can identify your Mac via the steps I have shown you, though if you use Lion, you click on System Report to get to the "Model Identifier". I guess you have a MacBook Pro purchased in 2011, and you have problems with your actual trackpad (you didn't really confirm or deny it) while the MBP is located on a glass surface.
Either you contact Apple and make an appointment with the Genius Bar or ... I don't know really, as that is one of the strangest problems I have seen in a while, as the trackpad can't sense a glass surface on its bottom.

 

Just to confirm again: We are talking about the built in touchpad that sits below the keyboard on the laptop, right? The thing that is shown in the image shown above in Simsas post?

Can you post a picture of the glass desk and it's surroundings?

Have you tried moving the desk to a different location?

What happens if you connect or disconnect it from the charger?

The touchpad on the mac is capacitive - it might react to some very strong electric or magnetic field, e.g. from a nearby speaker or a bad power outlet... although I can't really imagine this being the reason.