So Mark, what are you using that's only drawing 20VA? That's pretty low power for a server type box - I assume it's a firewall of some description?

What Peter said about PCs being generally fairly resistive (i.e. close to unity power factor) is true. I just measured a normal P3 1GHz PC here (the company I work for designs and manufactures solid state energy meters so I have some background and the equipment to easily test) and the power factor was around 0.97 (12 degrees phase angle). So you're looking at 120 x 0.17 x cos 12 = ~20W. For all intents and purposes the power factor makes stuff all difference in this case.

The other way to work is to go backwards from the DC output and assume roughly 75-80% efficiency in the PSU. i.e PSU output / 0.8. Does that line up?

I just wish I could get my MythTV box down that low. It currently draws around 120-130W. XP2400 and 3 7200rpm drives doesn't help of course...