Cleaning up time machine
I'm not sure you need to delete old backups (although maybe it depends on what you mean by saying the the disk is "too full"). Time Machine is designed to fill up the drive with backups and then start deleting old backups once it is full.

This Apple support page says:


  Time Machine keeps hourly backups for
  the past 24 hours, daily backups for
  the past month, and weekly backups
  until your backup drive is full.


Time Machine may prompt you when the disk is full to say that it is starting to clear down old backups but this is really to inform you that old data is being deleted rather than an error. 

According to the page linked above, it is also possible to delete all the backups of a particular file or files from the Time Machine interface by right-clicking on them and choosing "Delete All Backups of..." from the context menu. This information is in the section right at the end of the page.