Couple of clarifying questions:
1) How long are you waiting?
2) Have you upgraded the OS to Lion?
When I upgraded from Snow Leopard to Lion, the first few time machines took hours 'preparing to backup' before the actual backup to occur. We're talking 6+ hours. I'd usually leave it over night to backup. Even my last fresh reinstall of Lion took about 20min to prepare the first time. Now the hourly backups take maybe 1min total.
My suggestion is to leave it over night and see what happens. It very well could be that the differences between your last backup and current machine state is so different it's taking a long time to determine all the changes.
Thanks a lot, this worked - I waited an hour or so and finally something happened!
However, my comp has something like 490gbs used and my ext Hd is now empty, and when I try to do the backup it says time machine needs 580gb!
How can that be? Surely it just needs exactly what is on the comp, ie 490 gbs