It's not a great exchange rate, but you can transfer up to 50,000 United miles per calendar year into Diners Club points at a 1:1 rate (note that if you do it in the opposite direction that rate is 2 DC points gets you 1 UA mile) and then transfer Diners Club points into various hotel accounts at the following rates:

2500 Diners = 750 Hyatt
2500 Diners = 750 Starwood
2500 Diners = 2000 Hilton
2500 Diners = 1500 Marriot
2500 Diners = 2000 Priority Club

I believe that with some point transfers if you pay a fee that transfers can be done on an expedited basis.

I also think that you might be able to use American express membership miles in a similar way.

As for direct transfers, alas, Hilton had a good exchange rate with UA, but canned the exchange after UA went into bankruptcy. I think it still has such an arrangement with AA and a few other American airlines, though.