My 5 cents on this: if you assume UA is still in business in May 2009, the safest way to go is to book them on UA metal with UA miles. I know others will say the opposite. However I'm going through nonsense with UA, AC, and LH right now because AC discontinued a flight. I had a revenue ticket on AC DEN to MAD via YYZ, and booked an award ticket through UA on the same flights for MrsXS.

AC took care of me, by offering me a number of AC codeshares on LH and/or JK to go via FRA. However, they couldn't do anything to make UA have award availability on the LH FRA-MAD they put me on. UA wouldn't/couldn't (see the gazillion posts on Starnet filtering, LH hard to find, etc.) get her onto the LH FRA-MAD. AC told me that if UA needed AC to open up space, UA could call AC and they'd work it out. UA told me they'd work to open UA space if needed. And, if LH had been in the itinerary in the first place, they would go to LH and work out space. But because of the different mix of partners originally, neither UA whose miles I used, nor AC, whose change caused the problem, could do anything to get MrsXS on the same itinerary.

Between now and May 2009, I expect many airlines to discontinue flights, make drastic schedule changes, drop entire destinations. If you book an all-UA from UA, UA has motivation, and control, to make things right if at all possible. If you book UA miles onto other partners, and have a situation where one partner's schedule change blows your itinerary and the only way to fix it is to pick another partner's flight, you could get stuck if that other partner doesn't have inventory.

If your concern is about UA actually rescinding your ticket, no, I don't believe that would happen under any circumstance where UA is still open for business and flying (even in bankruptcy again). However, if UA is in bankruptcy by then, other airlines might not honor their award ticket. This didn't happen in the past, but on a possibly 2nd pass through BK it might. But UA isn't going to pull your ticket on an already-ticketed trip, even if they raise their mileage award levels or add blackouts or any other type of change to the program