I was going to argue that no, it isn't, because it's not between two distinct slices of bread like a sandwich would be, but instead is nestled inside just one bun that's been sliced up a bit.
But then by that standard, a sub sandwich, which is nestled inside bread in a similar fashion, would not be a sandwich.
Hmm.
So if a sub sandwich is a sandwich even without the distinct slices, then by that logic, technically...yes, a hot dog is a sandwich. It's a filling of some sort between bread.
Still doesn't mean I won't look at someone a bit funny if they do call it one, though.