Interesting thread.

We are looking into one page vs. multi-page checkout too and for the same reasons as the original poster.
I guess there is a reason for the (insane) checkout process on sites like Amazon.com/Amazon.co.uk and a lot of other large e-commerce sites.
Personally I really like the one page checkout from UberCart, but there is a problem with enabling/disabling payment options based on delivery information, and also payment taxes which is based on chosen payment method.
Our solution right now is to print out a message telling the user that this or that payment option is not usable with selected delivery country - not an optimal solution.

I do not believe that a well implemented multi-page checkout would have much negative effect on sales:
- Basket >>> Name/Billing/Delivery information >>> Payment >>> Review Order >>> Order Complete
This compared to the current implementation:
- Basket >>> Name/Billing/Delivery/Payment Information >>> Review Order >>> Order Complete

The one page vs. multi page is a little academic, since the latter is not really "One page". The multi page is just one more click/step.

One other challenge which just hit us is from a customer request for a selectable delivery location by using a third party map (like Google Maps), where you select your closest post office or the post office you want to have the order shipped to (this affects payment too). This is included with a iFrame. A multi-page checkout would make this a lot easier to implement.

I'll do some brain storming here at the office and maybe we could bounty this to get things started.