[jaxn]

I think so.

I am not HTTP expert, but my understanding is that a 301 is a RESPONSE not a REQUEST.

What is most certainly happening is that Twitter is making a HEAD request to URLs to see if a redirect is happening. The http redirect is a RESPONSE of type 301 (permanent) or 302 (temporary).

And yes, I suspect this is happening somewhere that can be cached. It would be easy to tell by setting up a 301 redirect, posting it to twitter, and watching the server logs.