How to "middle-click" on new macbooks

Check out BetterTouchTool. It adds gestures to your trackpad or Magic Mouse and there's tons of gestures available to set actions to. Best of all, it's free!

I don't believe its configurable within the OS, but MiddleClick, gives you a 3-tap middle click. Works really well in Firefox, which is what it was originally designed for. You'll need to tweak it so it fires up on logon.

If you use Firefox, Command-Click will open in a new tab.

Or you can hold two fingers on the trackpad and click; this gives you a right-click. You could then select 'open in new tab' from the context menu. (or get an extension like Tab Mix Plus and make right-click open a new tab)