PHPKitchen has an interesting post covering one of the banes of most web developers existance - the "back button problem".

In searching around for solutions to the back button problem, I came across quite a useful article which discusses various approaches to this problem. Too often web application designers attempt to disable the back button, or simply refuse to support users who use or rely on the back button.

It doesn't have to be that way. Disabling the back button is ultimately impossible; it is far better to design for and live with the back button.

The article goes into more detail, on what can be done for different situations. It gives some quite useful ideas for helping you and your site's users get along...