I had the same situation. My first iPhone had a bad home button, and my second one had a sticky silent switch. My silent switch was sticky straight out of the Apple Store. If you still have a warranty, or have AppleCare, they'll replace the whole phone. Give Apple a ring at 1-800-SOS-APPL (1-800-767-2775 in the US) and explain that your switch is all of the sudden sticky, and that you didn't spill any liquid on the device (Or, I'd hope so!). They'll make you a Genius Bar appointment. Then, tell the Genius that it's all the sudden sticky, and they'll replace it. Be sure to do an iCloud backup, first, though.