You might have heard that Maint Plans has had some work on them in sp2. One enhancement is that the Maintenance Cleanup Task now allow for removing of backup files based on hours. This can lead to undesired too early cleanup if you upgraded such a plan. I haven't tested this, I just learned it from the open newsgroups, but my guess is that perhaps the smallest earlier interval (days) is not treated as the smallest interval (hours).
Whether or not that is an accurate description from a technical viewpoint, the end result is that your cleanup can be done earlier than expected.
The fix is simple: open the cleanup task, adjust the value and save it again.