This is a great move footballwise for Philadelphia.
Their offense is already built around having a quarterback with mobility (McNabb). McNabb is an aging QB with injury history and a godawful backup. Vick could fill in seamlessly using the same basic playbook if McNabb should go down. Makes perfect sense for Philly to grab him.
If McNabb is healthy and they use Vick at wideout, they have Jackson, Maclin, and Vick for a speed wideout corps, and Baskett and the TE group as red zone options. Not a bad offense if they can harness that athleticism, and they should with a month before the season.