could the pinion gear at the end of my speedometer cable going into the transmission have worn down just enough so it would not catch?

 

Why don't you take it out and look at it?

 

Just take the bolt off the retainer and the thing slides out. Goes back as easily.

I've never seen one that's been damaged, but it would be possible if the cable froze. However, the speedo cable isn't very strong and it would probably have to freeze/seize close to the gear to damage the gear.

But, Chris is right - look to see.