I've noticed that when an application, such as iChat or Twitter for Mac, sends a link to Safari it opens in a new window rather than a new tab. I can manually force it to open in a new tab by holding down cmd, but I'd like this to happen automatically. I don't recall links opening in new windows in Snow Leopard.
To be clear, I'm not talking about opening links within Safari itself, I've set these to open in new tabs already. I'm talking about external applications sending links to Safari.