Great question, and I want to know the answer, too!
By the way, you CAN have the HR10-250 (HD TiVo) do 1 SAT in with 2 OTA tuners... you just have to trick it by telling it you have 2 SAT lines hooked up. WARNING: This requires CONSTANT babysitting of the To Do list. I only do this because I also have an HR20 in the same location and do most of the SAT recording on it:
Currently I have 3 SAT lines running to the basement. The line going to the HR10 has OTA diplexed in. When going thru setup, I chose 2 SAT lines. If you only choose one SAT, then TiVo only enables one OTA tuner. The tricky part about lying to TiVo and saying that there are two SAT lines connected is that the [normally wonderful] TiVo scheduling brain will schedule two SAT shows opposite each other and not warn you. This is why you have to babysit the To Do list, to make sure that no SAT shows are conflicting. It's not too hard, as the channel numbers for OTA shows are very apparent (they have a hyphen) so everything else is SAT (check the program lengths if unknown!)
It also is a good idea if you only have one SAT connected to pre-tune the good tuner to the right SAT channel before it's time to record. Not so feasible for those long trips away from the house. For this reason as well, you should disable Suggestions (which I miss) as you don't want TiVo changing the channel or recording something on the good tuner when your programmed SAT show airs.
This is all still worth it for me b/c I usually have more conflicts between OTA stations than I do with SAT channels, so the 2nd OTA tuner is a must. You may ask, why don't I take another SAT from the HR20 and give it to the HD TiVo? Well, that would render that box a one-tuner machine, reducing me to 3 tuners total. As it is now, I have 4 tuners possible at any given time, of which 3 can record SAT. Your situation may make this setup unworkable.
If I find out that the HR20 operates similarly to the HD TiVo with 1 SAT connected but 2 tuners chosen, I probably will flip the operation of these units, as the 20 is purported to have a better OTA tuner and with 2 SAT lines on the HD TiVo I can go back to Suggestions. Of course, I can split the OTA to feed both.
Oh, yes... to answer your other probable question: Why not add a 4th SAT line? It would probably require a whole new run from the dish and getting it into the basement would probably mean new holes, which I don't want.
