I would like to find out information about developing an ecommerce
site using Flash. Flash would be used on the front end exclusively,
not just embedded Flash movies. A flash based shopping cart would be
needed. The shopping cart would be proprietary or if pre-existing
components exist, those could be leveraged.

Is it significantly more difficult to develop an ecommerce site in
Flash with a Flash shopping cart than say, PHP? If so, why? What are
the considerations/where are the issues?

The biggest issue I've found is that e-commerce sites must usually
have the "back button" enabled for the user to go back and forth
between purchases and/or products. If you don't wire the "back"
button, everything might be (or probably will be) lost and have to be
reset when the site is re-entered.

While you can wire the back button for IE and Netscape, it's a little
trickier for Mozilla and nearly impossible for Opera.

There is another wrinkle, too. In order to wire Flash for a back
button, you will have to use some form of frame or iFrame. Doing that
in Flash will disable the "POST" function in the Flash player, so
everything you do will have to be done using "GET". That opens up
another can of worms.