From an interface designer perspective I am quite torn. I really do think the "opening" panes as they are now are not working at all - they are confusing as hell to a user - they basically unite the worse of both worlds. As with single page checkout - yes that makes loading times less and make the user not think "oh there is so much more" but its a huge amount of info on one page - and I know for fact that when I showed that to people they where actually totally turned of to complete an order - in that regard I think a multi-page checkout is the way to go (and I though I had this going already for an early alpha version ??). It provides clean simple steps to proceed. What is important is that you show the user where they are currently and what more is expexted of them - like in a step by step breadcrumb or a list way. I see this more and more all over the net and I think this is the direction that makes most sense. Little information overflow - little unknown territory. (I studied communication design with interface design slapped on - not that this makes me an expert but I was taught what to look out for). As people are torn on this topic I would say it probably needs to be an option - but as said the current collapsing panes are really bad - they barely give you a glimpse on what is needed, they overfill the page with info, they move mouseclicktargets (the worst thing you can ever do to a user is move mouseclicktargets - it flat out frightens them) and they need javascript (brrrr).....
From a webdesigner standpoint I also would go for the single page one as they avoid scrolling the page, give you nice little information to style around and give you a clean way to work with...

just my 2 worthless pennys of course.....