I've been shooting with the 5d II at night during all receptions and using higher ISO and bringing in more ambient light. One of the issues that always presents itself is the fact that its so easy to use a slave flash to light up everything under the sun (when there is no sun). This technique is fine, except it tends knock down or take away any ambient light that existed. Blast em doesnt = better.
What we are doing these days is using Neutral Density Gel to actually take the power of the flash down by 1 or 2 or sometimes, 3 stops. I'm shooting OLD Sb-28's that are already at a lower power level than today's Sb's (800 + 900). Or the 580 EX II. The 28 simply is an older unit that I can dial down to 1/64 power. BUT, thats too bright for most of my stuff, considering I'm usually shooting to bring IN that ambient light, make some sort of atmosphere out of the reception vs give it atmosphere by dominating flash bursts. Make sense?
Before I get into MY own images, Id like you to spend 30 minutes reading this entire blog post. My friend Dave from DQ studios recently posted a blog post about off camera flash and hinted on this very subject. The main problem with these flashes is they are just too stinking HOT. In order to preserve the mood and save the ambiance in the room, we must turn our flashes WAY down. Dave spent ALL day writing this blog post. Dave and Quin are pretty stinking HOTness behind the camera coupled with off camera flash work. Check out his thoughts:

Nikon D700 + 105mm f2 @ISO1250, f2.2 & 1/100. Off-camera flash: Nikon SB900 triggered via Quantum FW10w Transceivers
Dave writes:
Yesterday I was talking with a marketing director of a flash company about flash needs in the 21st Century. We all know we love balancing and, at times, overpowering sun with flash. But how about lighting with off-camera flash in low-light/high-ISO surroundings?
There’s lots of relatively high-powered flashes made for bright shooting and overpowering dark scenes. What’s lacking is a professional strobe with *really* low-powered output for accenting low-light environments.
“We’re not in Kansas anymore …”
For sure, click here to read his full article.
On the same subject, but outside on the beach, I still must preserve what ambient I've been given. In this case, I had a dance floor and some stage lights but also, mixed with dim dim tungsten lights in the trees, and many other low power light sources.
I've posted a few images here to give you and idea of what I'm talking about...

This one was shot at ISO 1000 using 2.8 shot at 1/13 of a second. I pulled a little movement off by zooming in on the lens to get the floor moving. The flash with 1 ND filter, set to low power (1/64) is blasting away at the girls on the floor. The slave flash acts as my shutter, freezing action, letting everything els

Shot a 1/25 of a Sec. I little camera movement from left to right so the ambient starts to jump off the page. My flash becomes my new shutter



This just a scene setter shot of the dance floor with a very low power flash giving me some side light, keeping just enough on them not to take away the mood the light from the dance floor was giving off. Perhaps a little too hot on the flash...
This one should help you in a big way :)
cheers,
matt
Mateo,
Great shot. One thing I’d like to point out is the “quality” of sharpness from the flash. I’ve found a fill strobe will give you charity like your example here. I particularly like the subtle depth of field fore and aft. Excellent example of specific sharpness.
tailwinds~
mike
Very educational, thank you for sharing!
Useful and interesting. Always like your post.