"I've read that rumor in other threads but so far this is the only article with a quote from the Capitol Police."
Its being echoed around both left & right sides of the blogosphere [Kos, Powerline, etc]. But you are correct, I can't find "evidence" of the banner yet.
Frankly, it would bother me even more that she would be able to smuggle in a banner. One would think the Capitol Police would recognize a know activist like her.
Reuters & CNN have it:
"CNN said Sheehan was arrested after unveiling a banner and was being held on Capitol Hill for about an hour, which would keep her away from the address, which was set to start at 9 p.m. EST."
http://today.reuters.com/News/newsArticle.aspx?type=domesticNews&storyID=2006-02-01T021336Z_01_N3129628_RTRUKOC_0_US-BUSH-SPEECH-SHEEHAN.xml