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The Tablet version of the Lenovo X61 is a bit larger, but is still quite portable and a compelling device.
There are a few modules available at launch, such as a display, digital camera, GPS, and accelerometer. There are 81 modules under consideration and with the fully hackable nature of the hardware more modules will probably be coming from the public.
The Bug Labs modules are plug and play and are a mobile geek's playground. There are some reduced prices now for early adopters as well.