Not long ago, Mr.McNally blogged about High Speed Portraiture. That post co-incidently followed a quick shoot I did using Canon's High Speed Sync (HSS) of the lovely couple below.
For this shot, I had to concern myself with:
a) Not blowing out the background on a bright morning.
b) Not seeing too much detail in the background.
To get a nice balance ended up at ISO 100, f/3.5 (for blurry background), and 1/800th (for no blown highlights)... Beyond normal flash sync speed.
Luckily, I've got a nice 3 metre (9 foot) home made OC-E3 (off camera cord) for my 580EXII, so I could get that off camera, in a nice cross-lighting position... by turning on the HSS feature (it's just a button on the back of the 580EXII that has a lightning bolt next to an 'H'), I bumped up to full power, bounced into a reflective brolley, moved in as close as it needed to be to still be out of shot, and pumping out enough light to get a nice exposure.
Why full power? As Joe mentioned, the downside of high speed sync is a big loss of flash power. And when you bounce into a brolley, you lose even more.
Gotta love long off camera cords a.k.a. strobe on a rope.
Also good for the occasional dramatic sunset