Thanks to both of you for replying! And I apologize for not updating sooner, but a new baby complicated things!
I ended up doing a very thorough check with a smaller TV, for as many combinations that I could think of, and did narrow it down to the receiver (I don't know if I can do firmware updates on this one? There are no Internet connections to it, and nothing in the manual...). So I found a local electronics repair shop and brought it in, and in the meantime went strait from my cable box to the TV.
This worked well for about a day... then that HDMI input stopped working, moved to #2, worked for a while, #3 and #4 wouldn't work at all. Lo and behold, the repair guy calls me back asking if anything else wasn't working. His best guess was a power surge. All of my A/V equipment in the closet is on surge protectors... but my TV, where I've been meaning to move the plug up behind the TV to finish off my "wireless" home theater setup, wasn't on a surge protector anymore! So what must have happened is a surge hit the TV, backtracked to the receiver and stopped there...
I'm now debating to take the TV in for repair (Vizio SV421XVT), or purchase a new one in my current budget range (Vizio E470i-A0).
If anyone has any thoughts on that I'd be grateful!