First, HDMI is not better than component. If you use good cables, component will look just as good. People somehow think that HDMI will look better because its all digital, and while it reduces the odds of signal noise, the picture itself is no better than a component signal on a good cable which is more resistant to noise.
That said, Component to HDMI, possible.
HDMI to component, no way in hell!. HDMI signals carry digital copy protection which only allows the signal to pass when connected to an approve device when the copy protection flag is set. Otherwise, the signal is blocked to prevent someone from recording the signal. Such a converter would either filter out the copy protection which would be illegal in the USA under the DMCA, or the thing would block the signal when the copy protection flag is set. If that flag is ever set on HD-DVDs, then this device would block those DVDs from being viewed on your xbox-360.