Jason recently wrote up a simple recipe to allow membership checkout in a modal window. This allows you to create a button (in any page, post, widget) that will instantly display the membership checkout form. Read on for more information, how to set it up, and some notes.
Don’t Show the “Confirm Email” or “Confirm Password” Fields on Membership Checkout
If you’d like to tighten up your checkout page, the code recipe below has filters to hide the confirm email or confirm password fields on membership checkout.