This looks interesting: PhiloMetron has developed a small adhesive patch which employs a variety of sensors to estimate your caloric intake. It then transmits the data via Bluetooth so you can view the trends on your cell phone.
While you’re waiting for that to become available and/or affordable, you might be interested in FatWatch, an app for your iPhone that also estimates your caloric intake, although it requires you to step on a scale and make a note of the number once a day.
(P.S. I found the above news via The Skeptical Hypochondriac. Recommended if you’re interested in health news without the hysteria.)