One page checkout was an active topic on facebook and more active behind the scenes at AbleCommerce. Unless you have a REALLY simple store one page checkout doesn't improve the checkout flow.
With the exception of a bit of bold text you'll find the unedited facebook comments on one page checkout below:

11Like · · Share
George Jaros We have clients that like both checkout methods. We find it easier to integrate customizations and adjustments to the checkout flow with the multi page checkout. The biggest complaint we have from clients though is that the one page checkout isn't really a one page checkout. Technically it is, but to a customer they still have to enter billing and shipping information, click continue, then enter payment information on a second screen. A true one page checkout, like what sites such as Cafe Press have, would be a huge plus in our clients' eyes.
February 21 at 8:02pm · Like
AbleCommerce Cafe Press has a 'fixed' business logic for shipping charges and other items, which we cannot assume in our checkout logic which requires dynamic updates of information based on shipping location for example.
February 21 at 8:08pm · Like
Neal Culiner Pick one and do it right. There is no perfect answer and I don't think consumers care. One page may be information overload. Breaking it into logical steps will prevent order errors.
February 21 at 8:09pm · Unlike · 1
Charles Alvis Personally I think one page checkout is 'over-hyped'. I've never had a customer tell me they did not order because I didn't have one page checkout. One page checkout is good for impulse buys, but if you want to maintain a relationship with the customer (Not to mention it's easier to modify) I think multi is the way to go.
February 21 at 8:34pm · Unlike · 1
Joe Payne I think history has proven the concept of an all-inclusive checkout page is simply not practical. Stability is paramount to all other needs in the checkout portion of a website. No amount of features will make an unstable checkout appealing to any business or shopper.
February 21 at 10:39pm · Unlike · 1
Mike Stephenson would like to see other options such as amazon checkout
February 21 at 10:52pm · Unlike · 1
Chris Frank I like multiple steps, I would prefer only one checkout option. However in the end, I want it to be successful, I suggest doing what the "big boys" are doing.
February 22 at 1:04pm · Unlike · 1
Sammi May I would like to see some type of enhancement, that if an item is not in stock, and we are taking a back-order. That this info is present to the customers consistently through the checkout process and the order confirmation. It seems somehow, they always miss the disclaimer on the product page.
February 23 at 5:27pm · Unlike · 2
Jim Sewell We use one-page checkout. with great success, because, as Plugables says in the forums:
"Multi-page checkout does not support anonymous checkout as far as I know. Only One-Page-Checkout supports it. So I assume you will need to customize the multi-page checkout files to achieve what you want."
All of our purchases are anonymous so if you do away with one-page it would be really great if multi-page deals with anonymous checkout properly.
May 7 at 4:26pm · Like
AbleCommerce Jim, thank you for the feedback! We'll be sure to take that into consideration.
May 7 at 5:13pm · Like
David O'Leary On a related note, I've been working on customizations to both styles today and more attention should be focused on better encapsulating pieces within User Controls. This would make it much easier to provide several options for checkout. For example, currently, there's no common Address control and the related code is repeated in several places which make it difficult to modify. Same holds true for Login/Registration. If the code for checkout were better encapsulated, it'd be much cleaner to make improvements and to offer a few different options for checkout work flow.
June 12 at 8:42pm · Like