We've recently had to block non-gadget IFrames in order to protect our
users from security risks. We understand this may inconvenience some
of you, but please note that you can still create or use gadgets to
embed external content.

For those of you worried about serving ads on your Site, we are in the
process of fully integrating AdSense into Google Sites.

Finally, we will enable IFrames at some point in the future, once we
can ensure security for all of our users. Thanks for your patience and
understanding.

Dana
The Google Sites team