Replying to [comment:2 scott.gonzalez]:
It doesn't seem like like a good idea to have the tooltip move on its own after it has been displayed. Why are you not using a placeholder that approximates the size of the actual content?
I agree that moving the tooltip around after it has an established position may be jarring, however:
1) tooltip provides 'position' options which should be applied consistently and be documented when they are
not and for what reason.
2) when the trigger is near the bottom edge and the tooltip is big enough to be positioned above the
trigger the new content will have a collision with the trigger anyway because it isn't repositioned higher
rather, e.g. http://jsfiddle.net/vqvHW/
In this particular case adding an approximation is difficult because the new content varies, i.e. sometimes
multiple lines and sometimes an error message - in that case the tooltip is too high in the first instance
and then has a weird gap and looks broken.