The hovercard idea is very nicely done. Just a tiny bit of copy-in Javascript, and instantly they hook into any @name on your pages. It's easy for a site to add, looks good and makes the connection between Twitter and the @name look even stronger.
That button looks just like Facebook Connect.
I don't mean to be too flip, but isn't that just the official (probably O'Reilly trademarked) Web 2.0 look for buttons? They all look like that now to me.