You can have a look at an example I just created for this thread.
It took me a couple of hours to do it.
Have a look.
it's all done with AE PB 5.5, without the use of any external softwares or plugins.
Few things I did :
I used few Fractal Noises with Twirl dynamics animating different complexity values. I also made sure I used two layers for the text and slighly offset the speed of the motion for one of the layer to give the illusion of smoke trail. I played a lot with the gaussian blur, and add a suddle directional blur.
I also scaled the layers to give the sensation than the smoke was taking more space over time.
I add a slight twirl to the overall image.
And I made sure than all my motions were smooth for the IN and OUT points.
I think it looks almost like water. In a professional environement I would have use 3D particles or stock footage or Illusion (http://www.wondertouch.com/default.asp)
There is a lot of pre-comping, and it's somehwat processor intense.
AE 5.5 PB FILE