Requires the device to be online and registered to your Google account.
This technique has been confirmed to work for thousands of users already, but does not work for every device type and configuration so your results may vary.
Please take the time to read the instructions carefully:
1) Unplug the charger from your device.
2) Login to the Web based Android Market using your PC.
3) Using the Web Market, install this application to your registered device.
4) Once installed, plug your charger into the device to activate the bypass.
5) Once bypassed you will see an "Activate" button. Click it, and your screen lock will be reset.
Some newer devices will not detect the charger being plugged in, so I have included a second option, turning the device off (or pulling out the battery) and back on again will cause it to activate the bypass.
If the app does not work for you it could be that your version of Android is not compatible with this technique or you have a custom lock screen which cannot be bypassed. Unfortunately there isn't anything I can do to support those devices.
There are a couple of reviews stating that the app doesn't allow itself to be uninstalled after use. Rest assured that the app can easily be uninstalled, either through the Android system or the button in the app labelled "Uninstall". Most people manage it but if you have issues support is just an email away.