Basically you are not allowed to have other ads on the page that have the same look and feel as Google Adsense. I don't think it's the case for widgetbucks, but I haven't seen an official position about this. You could write the Adsense Team and ask, or maybe someone around here already did that.
That's what I was thinking, the ads are a totally different style and cannot be mistaken for eachother.
I have installed it on one of my sites and its a different colour to the adsense ads which have been blended to the colours of the site. It stands out like a 'sore thumb,' and just to make doubley sure I have added a horizontal rule above and below the widgetbucks ad.