Great news, you can now pay for your annual or monthly membership with PayPal!
Go to the upgrade page and select your desired membership. On the Checkout screen, you'll see the option to "Pay with PayPal". Click on that link and you're all set to pay via PayPal.
Just a heads up, if you have a special promo code be sure to enter it in the "Have a Discount Code?" box, before clicking on the "Pay with PayPal" link. This will apply any promotional offers to your Paypal purchase. :-)
Here is a step-by-step tutorial on how to upgrade using the pay with PayPal option: