Do we have to book all travel directly through UAL if we are trying to use our miles on a different airline - obviously a mileage partner. Specifically, I am looking at US Air for a route that United also covers. Thanks!

how else would you book it?
And I'd be a bit nervous booking a US Air w/ UA miles anymore....unless it's a flight happening soon.

I'm pretty sure you need to log into your UA account in order to use FF miles on a flight. If you do a one-way search you may see more flight options. I've had some very interesting routes to and from Asia by booking one-way trips. I understand some travelers would never consider booking two one-ways, but I've had some very good choices by doing it this way.

US Air has never allowed to book one way without paying the round trip mileage required.

If you are using Mileage Plus miles then yes, you need to book through Mileage Plus. When you search available award seats on US should appear. If not, you can always call the MP line (and pay the associated booking fee) and ask if they can find something on US. If you have specific flights you want be prepared to give that info to the agent.

Given the upcoming merger of USAir with AA and USAir's departure from the Star Alliance, suggest you get this booked soonest.

You will book your trip with United because you are using MP miles.

In April of last year I booked a United MP award trip that involved Qatar Airways. Two weeks later, Qatar left Star Alliance. IIRC, they announced that new award bookings on QR could be made up until some time in September. No changes after that.

That meant that I had to take the trip in January as it was booked without any date/time changes on the Qatar flights (date/time changes on awards are normally easy to do).

That potential for future inflexibility on changes would be a reason for being nervous about booking an award trip involving US Airways far in advance of the travel on US.

Thanks for the good advice!