glenn1, I knew what I was thinking, but the keyboard didn't follow! Of course you are right - the value of a zero will drop when rates rise. I'll blame my voice-recognition software - I said "If interest rates go up" but it typed "If interest rates drop". (Well, if I actually had voice-recognition software, that would almost be believable - doh!)
The other point I didn't make well (geez, what a crummy post that was on my part) was that even if zeros are available with that short of a term, it wouldn't be worth the additional risk. With a MMF, there is no principal risk even if he needs the money sooner. With a zero, if rates go up, he would be locked in until maturity to avoid a loss. Realistically, though, with a zero of that short of a term the volatility wouldn't be very large since the time to maturity is comparatively short. We're agreed on that, I'm sure.
Thanks for catching my error!