The only syntax that Google says is supported right now is the "(weight=ignore)" syntax. One would presume that at some point they'll tell us that we can set a weight value (which I'd guess is a value from 0.0 to 1.0) but I haven't tried it to see if it'll do anything or not. Remember that these are just HINTS to Google. They may disregard what you say.... especially if they catch you using these directives to do keyword stuffing.
When my blog sites get blog ads, it is usually when there is nothing contextually relevant for AdSense. Sometimes, though, I've made more CPC from blog ads than more relevant ads based on the topic. AdSense is very accurate for my content, and I like to look at the ads served for older articles. My average impression to unique visitor ratio is over 4 pages now, which is very good -- my CPC goes up as my visitors head deeper into my site.