I recently swapped a HR10-250 for a HR21, per DirecTV's MPEG-4 upgrade program. A few days later I called them up stating I could get OTA with the old HD-Tivo, but not now with the HR21. The CSR balked a little at first, but I pointed out that I wanted to be able to at least watch locals when it rained enough to knock out the sats, and watching OTA from nearby markets. The kicker for me, however, was probably the fact that at the moment D* only provides two of my locals in HD, but I can get the rest in HD only OTA. I guess there can be advantages to being in a smaller market!! Anyway, in the end I was able to get the AM21 shipped to me for free.
Hope that helps.