I migrated from Macbook White to 15" unibody Macbook Pro and the first TimeMachine backup from the pro was painfully slow. It was a few kilobytes/sec and I had 27GB to backup!. The external harddisk was connected over firewire and the previous backups were always fast. I could browse the disk and also transfer files from it really fast, so I knew it was problem with TimeMachine.
According to some forums deleting the InProgress file inside TimeMachine folder helps [DON"T DO THIS] but that didn't do the trick.Frustrated , I began searching again and landed up on this post, which identified Finder as the culprit. I looked at my "Application Monitor" and Finder was eating 60% of my CPU. I quit Finder to see Dropbox eating away another 30%. I quite that too for good measure and suddenly the backup speed went to 2MB/sec.
The reason I warned about not deleting the InProgress folder is because it landed me in a pile deep of shit. When you delete the folder it goes to the Trash but you will not be able to expunge it from there. Clearing Trash will hang. This is because of strong ACL protections on the files. There was no way I could turn off the ACL protection [fsaclctl is absent in Snow Leopard and the one I copied from Leopard seemed to work but it didn't. chmod -R -N also failed spectacularly] to trash the file or delete a new one which was created. I had to install Macdrive on windows and connect the drive to a windows machine to delete the folder. Even then it was fraught with errors. 3 hours of my life I will never get back :(