Thanks everyone. He's still missing, unfortunately, and I am SERIOUSLY worried. But we think we know what happened.
There's this pack of stray dogs that run through the woods, and according to one of our neighbors, they hang out behind her house, and that's where Dexter also likes to hang out. The dogs have gotten after him before, and... I'm worried he may have been killed by the dogs.
We WILL be going to our local Humane Society/Animal Control sometime Saturday, though, and hopefully he was picked up.
On a happy note, however, we found out who we got him from was wrong about his breed. He is not a Siamese/Egyptian Mau mix, I don't believe. I think he's a Lynx Point Siamese, as he looks EXACTLY, to the tee, like them. So if we find him, at least we know for sure what he is.