According to the UA Advertising page, there isn't a slogan at this time:
With the merger of United and Continental in 2010, the airline created a new advertising campaign designed to speak to customers about the new airline, highlighting the look and feel of the new United and delivering important information about the integration of the airline’s operations.
In the coming months, as United and Continental begin to fully operate as one airline, there will be a great new advertising campaign….we hope it speaks to you and inspires you.
I read this to mean that we'll see a new slogan about the same time we see a redesign to the website, at the end of this year.