Paypal are conducting scheduled maintenance late tonight (PDT) for a 1 hr period at the following time:
10PM - 11PM PDT
12AM - 1AM CDT on 20/04/12
1AM - 2AM EDT on 20/04/12
6AM - 7AM BST on 20/04/12
During this time, Checking out via Paypal will be unavailable. I'll also be posting on Etsystatus (etsystatus.com, or @etsystatus on twitter) when the downtime starts / finishes, and additionally will reply to this thread to let you know when Paypal checkouts are enabled again.