I am getting this now too. Was getting it all yesterday and now today. When I upgraded to 10.5.2 it worked fine but now it's borked. Manually running it doesn't help either.
I had exactly this problem and posted about it in another thread.
I have an Intel iMac with a 500GB internal disk and a WD 1TB My Book Studio external disk for Time Machine backups.
I could open the external drive and see all the previous backups and drag files across to it. but Time Machine kept failing (spent about 1/2 hour saying Preparing, then exiting with an error message
Unable to complete backup. An error occurred while copying files to the backup volume." )
I started to type up an email to Apple Support, and in doing so, realised that the used space on my internal disk was just a bit greater than the free space on the external disk.
So I deleted a number of large files that I had already backed up (and didn't really need anymore anyway). When the used space dropped, Time Machine started backing up.
The first backup after the upgrade seems to be nearly the whole disk (my backup was 212 GB which isn't the whole size of the used part of my internal disk, but which is much, much more than a delta (I had added about 8GB of files plus the 10.5.2 update since the last backup).
So try deleting (or backing up first, perhaps to DVD, then deleting) until the free space on the external drive is greater than the used space on the internal drive and try Time Machine again.
Kerry