The main differences include a lack of an Ethernet port and the associated networking chip as well as there only being one USB port instead of two. There was originally going to be less memory on the Model A (128MB instead of 256MB), but the Raspberry Pi Foundation managed to make enough cost savings during a redesign to increase the amount to 256MB on the cheaper version.
With all the focus being on the Model B, we haven’t actually seen the (near) final Model A board yet. But that changes today as Eben Upton has just shown off the $25 board."
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