The back button issue with Ajax is a big stumbling point for our clients. On the one page checkout it also might help to add a has to the url so we can get a better accurate view of where people are dropping off since the url stays at default.aspx.
Here are some links on how it is done.
http://code.google.com/p/reallysimplehistory/
http://quickstarts.asp.net/3-5-extensio ... erver.aspx