and if they go back 3 pages and do somethign different so that they are using the session ID from 3 requests ago, are they just out of luck?

or, to be a little more explicit,

action	sends session_ID
user comes to site	n/a
server serves main page	n/a
user logs in	n/a
server shows logged-in page	123
user requests page 52	123
server gives page 52	456
user requests page 52-c	456
server gives page 52-c	135
user requests page 52-f	135
server gives page 52-f	237
user hits "back" to go back to login screen and requests page 38	123
server was expecting 237 ... what does it do?	?

this case, and others like it, make me think this method is unworkable. but maybe you see it differently; maybe i'm overlooking something, or maybe you just are willing to do more work than i am. but if it will continue to accept old session_ID's, then why bother changing them? and if it won't, i think a lot of people are going to be upset at the inability to "back".