It also happens on my computer with XP SP3. See the section The Computer Hibernates After Resume in this document for an explanation:
After your computer resumes from
hibernation, it may enter hibernation
again after five minutes of
inactivity, regardless of the settings
you have configured in the Power
Options tool in Control Panel. This
occurs if the computer's BIOS resets
itself as the computer resumes from
hibernation and clears any wakeup
signals that might be active.
Therefore, the operating system cannot
determine why the computer woke up and
as such returns to hibernation after
five minutes of inactivity. It does so
to avoid situations where an
application wakes up the computer and
causes the battery to drain while the
computer is not being used.
In other words, it's not a bug, it's a feature... Well, I would agree if it was possible to turn this behaviour off, but there seems to be no way except to reconfigure your BIOS, which in my case is not possible :-(