According to an Amazon press release today, the company has launched Guild Wars 2 pre-purchases. Users who buy the Standard or Digital Deluxe editions via Amazon will receive a digital download and pre-order key, beta access, pre-purchase perks, and $5 credit for Amazon video games.
Some of the details are a little hazy, as ArenaNet has made a distinction between pre-ordering (which is exactly what you're used to it being) and pre-purchasing (which is when you pay the full cost up-front) but Amazon appears to be using the terms interchangeably on the product page. Additionally, Amazon has not been added to the official list of pre-purchase retailers for North America (for that matter, Amazon has also not been listed on the official page as being able to distribute the Collector's Edition, but apparently it has physical boxes for sale as well).
Anyway, if you haven't bought Guild Wars 2 but would like to, and if the thought of $5 Amazon credit and some in-game perks is appealing, this may be A Thing for you.
[Source: Amazon press release]