Here is a WaPo/ABC poll, and one from CNN/USA Today/Gallup on the effects of Powells speech to the Security Council. They are oddly confusing, if not contradictory. Yet, Oxblog makes sense of them, in a few paragraphs. Here are a few good lines from Oxblog, the rest is also worth reading.
"What neither Gallup nor a WaPo storyabout the polls points out is how remarkable it is that both
Bushs State of the Union speech and Powells UN address significantly increased support for an
invasion. This sort of double-bounce has almost no historical precedents."