The reason the woman with the pro-troops tshirt on was not arrested was because she agreed to leave (at lease according to CBS-which unfortunately is the only place I've heard of why she wasn't arrested).
Cindy Sheehan did not want to leave. Peacefully or otherwise. Apprently they had to arrest her.
Here is a link to the article on CBS:
http://www.cbsnews.com/stories/2006/02/02/politics/main1272923.shtml
and the part about Young, the other woman:
"Sheehan was taken away in handcuffs before Mr. Bush's arrival at the Capitol and charged with a misdemeanor, while Young left the gallery and therefore was not arrested, said Capitol Police Chief Terrance Gainer in a statement."
It implies to me that Young left the gallery and they had no problem with her.