Apple - What is time machine backing up?

There are two tools I know of that do this: (1) BackupLoupe (which is a GUI application) and (2) timedog (command-line Perl script).

Mobile Time Machine and Time Machine in Lion: using Terminal to view activities

At any time

A command such as this will present a lightweight overview:

tail -f /private/var/log/system.log | grep -E 'backup|mtm'

During a backup

A command such as this will present detail:

sudo fs_usage cmd backupd

When you have finished

To interrupt a running command:

controlc

tmutil

From the manual page for tmutil in Mac OS X 10.7 (Build 11A511), the description:

tmutil provides methods of controlling and interacting with Time Machine, as well as examining and manipulating Time Machine backups. Common abilities include restoring data from backups, editing exclusions, and comparing backups.

Expect Apple to publish that manual page, and others for 10.7, at:

• API Reference: Mac OS X Manual Pages

TimeTracker is a quick-and-dirty application that displays the contents of your Time Machine backups, and shows what's changed since the previous backup.