In its analysis released Wednesday, ABI Research said that Microsoft (MSFT) faces an uphill battle in trying to enter the mobile computing tablet market with its own line of tablet computers. Microsoft unveiled the new tablet line, named Surface, on Monday. It includes a device aimed directly at Apple's (AAPL) iPad, as well as another, larger tablet intended to compete with lightweight laptops made by Apple, Hewlett-Packard (HPQ), Samsung and