Planning to fly on United or Continental at any point over the next few years? Then it looks like your odds of having in-flight WiFi will be getting increasingly better. United Continental Holdings announced today that it's signed an agreement with Panasonic Avionics Corporation to bring satellite-based WiFi connectivity to more than 300 of its aircraft beginning in mid-2012. Those aircraft will include Airbus 319 and 320 and Boeing 747, 757, 767, 777 and 787, and the company says it expects its entire mainline fleet to be covered by 2015. No specific word