Installed PB5 and then ran a TM backup. All it backed up was something on the order of 153 bytes of data.
Usually, after an OS X update, several gigs of data get backed up.
I think it is broken in some way. For some reason an external HDD for me is seen as a network drive, so backs up in a sparsebundle file rather than Backups.backupdb directory.
I had like a 7.5 GB backup after my update. Although I guess there could've been other stuff in there. But I don't remember doing much with my computer between my backups.