I discovered something today! At Harlequin.com, they now have books available a month before their release month for purchase. This is how it's been at millsandboon.com for as long as I've paid attention, but North America was still limited to release month.
Now, some lines have all their different versions available right now(Michelle Smart's March Presents is flipping awesome), but Medicals have only ebooks available. They will become available at other retailers in March.