I know a couple people that had woopra on their self-hosted blogs, and it messed up image uploading and the visual editor, and also kept pages from fully loading. I actually signed up, but have never installed it for that very reason.
I think someone will egt around to creating a fix for this sooner or later. As it stands the only way Woopra could work on WordPress if all the users were in subdomains.
There is a modified re-charged version of WordPress that appears to be using Woopra (if you watch teh bottom bar and see it on refresh), but it is still in beta and not open to public yet, I was wondering how they got Woopra on there? This site here: http://myjournal.com
If myJournal.com has it then why not WordPress.com?