Apple changed the face of mobile computing when it introduced the iPad. The evolutionary improvements in the iPad 2 solidfied its lead in tablets. But after a slow start, tablets running Google's Android operating system are finally starting to catch up. There are three new Android tablets that present serious challenges to the iPad. All have Wi-Fi (3G/4G versions may be coming), dual-core processors, 10.1 inch screens, and 16 GB of onboard storage to match the least expensive $500 Wi-Fi only iPad 2...