The relevant rule from the SRD:
If a flying creature fails to maintain its minimum forward speed, it must land at the end of its movement. If it is too high above the ground to land, it falls straight down...
The reason I raise this question is because the rules seem to indicate the answer is "yes", but on the other hand I can quite well imagine an adult dragon that gets Slowed mid-air to glide on and take standard actions while gliding?
I guess that leads me to ask this question in a rephrased way: Can a creature with poor or clumsy maneuverability glide as a swift or even free action?