If it's for snap shots or general mulling around it's always in AV. The aperture has the single biggest effect (when only using ambient light) of your 3 adjustments. It is a compositional choice as well as a technical one. As long as the shutter is fast enough to produce a sharp picture (Unless you're deliberately going for motion blur, but i'de argue that style is by far in the minority for most) you really want to be controlling your depth of feild. Leaving your aperture upto it's own devices is a bad idea IMO.
I fully agree with this.
When I'm doing my own shoots or if I've got the time to fiddle I will go full manual. However, with work, a lot of the time I'm doing press photography. You only get one chance to get it right and it's usually pretty fast paced / intense. For these instances I will go with Aperture Priority unless I'm using a flash. It's just one less thing to worry about.
My biggest drawback at the minute is that I'm using a D50 at home and a D200 at work and both of them are rubbish over ISO640.
I wan't one of the new night-vision cameras so that I can stop worrying about ISO.

Panzer