All: Please forgive my spelling as I am covertly responding from work....
Dave: Wow! I just picked up the 85 1.8 for portraits, but I may have to experiment with my 70-200 as well...
Kevin: I just wanted reassure you that I have had good sucess using the 580 to light up a 6x4 backdrop, but it will take a little trial and error to get the settings right... I find that placing it on the floor just behind the subject, and angled about 45 degrees up twords your backdrop works best depending on how much working distance you have.
If you are doing wireless, I have run into situations, where my subject actually blocked the transmission to the flash causing it not to fire. Another position that helps in thease situations is lying the flash on its back (with the infer-red reciever facing the cieling) and angling the flash as needed from that position.
But remember... Manual exposure on the flash! Any of the auto/ttl settings will cause it to compensate and light the backround differently every time. Also, auto settings will attempt to "properly" expose the backround in a way that will not work with the rest of the picture. It took me a few tries to get it right, but once you figure out what works best for you, you should be able to reproduce the same light as needed.
Also while I havent tried it yet, your suggestion second option about putting the bounve fill in front of the subject is exactly what I intend to do the next time i go out, because without it, there will be some shadows that you will have to readjust lighting to compensate for.
Finally (sorry this was so long)... I find that you do want to seperate your subject from the backround as much as you can allow. Otherwise, you may run into a situation where the backround's illumination could reflect back on your subject. A hairlight can help compensate for this as well.
I have never used studio lights for my jobs but I hear that the lights you have are supposed to work very well. As I mentioned, the only resion i went the all speedlight rout was to maximize portability, and not have to rely on finding available power at a shooting location.