Here's something I posted on the Hilton board back in January. You may find it useful when considering the Hilton Reward Exchange program for moving miles around.

HILTON HHONORS REWARD EXCHANGE PROGRAM
EXCHANGE RATE CHARTS

GROUP 1 AIRLINES:
Continental Airlines, Delta Airlines, TWA, United Airlines, Aero California, Hawaiian Airlines, LatinPass, Mexicana Airlines, Midwest Express Airlines, South African Airways

GROUP 2 AIRLINES:
American Airlines, Virgin Atlantic Airways

GROUP 3 AIRLINE:
LanChile

mi = airline miles, pt = HHonors points, km = airline kilometers


CONVERTING BETWEEN ALL GROUP 1 AIRLINES:

5000 mi -> 10000 pt -> 1500 mi 70% loss
10000 mi -> 20000 pt -> 3500 mi 65% loss
25000 mi -> 50000 pt -> 10000 mi 60% loss


CONVERTING FROM GROUP 1 TO AMERICAN AIRLINES:

5000 mi -> 10000 pt -> 2000 mi 60% loss
10000 mi -> 20000 pt -> 4000 mi 60% loss
25000 mi -> 50000 pt -> 12000 mi 52% loss


CONVERTING FROM GROUP 2 TO GROUPS 1, 3, AND VIRGIN:

10000 mi -> 10000 pt -> 1500 mi 85% loss
20000 mi -> 20000 pt -> 3500 mi/6500 km 83% loss (78% loss to LanChile)
35000 mi -> 35000 pt -> 10000 km (81% loss to LanChile)
50000 mi -> 50000 pt -> 10000mi/15000km 80% penalty (80% loss to LanChile)


CONVERTING FROM GROUPS 2 AND 3 TO AMERICAN AIRLINES:

10000 mi/km -> 10000 pt -> 2000 mi 80% loss (70% loss from LanChile)
20000 mi/km -> 20000 pt -> 4000 mi 80% loss (70% loss from LanChile)
50000 mi/km -> 50000 pt -> 12000 mi 76% loss (64% loss from LanChile)


CONVERTING FROM GROUP 1 TO GROUP 3:

10000 mi -> 20000 pt -> 6500 km 57% loss
17500 mi -> 35000 pt -> 10000 km 62% loss
25000 mi -> 50000 pt -> 15000 km 60% loss
(note that it's cheaper to move 10000 miles than it is to move anything more)


CONVERTING FROM GROUP 3 TO GROUP 1:

10000 km -> 10000 pt -> 1500 mi 77% loss
20000 km -> 20000 pt -> 3500 mi 74% loss
50000 km -> 50000 pt -> 10000 mi 70% loss


You can transfer as little as 5000 miles (10000 mi/km for Virgin and LanChile) at a time out of the airline programs and into your HHonors account without suffering the loss. That means that you can pool points from various airline programs in your HHonors account, and then transfer them out to one airline program to get the best exchange rate. But once you transfer the points out of the airline, you cannot put them back without suffering the loss.

You might also consider that using the points for a stay at a Hilton Hotel is a better value than transfering the points to an airline, especially if you stay at Hiltons anyway. Consider that if you used 20000 pts to stay 1 weekend night at a "Executive" class Hilton Hotel that would have cost you $100/night, you can use your points and get a value of 0.5 cents per point. If you transferred those same points to an airline, and redeemed 25000 miles for a ticket that would have cost you $400, you would only get a value of 0.32 cents per point. Thus, in the above example, you would be getting nearly 40% more value using the points for a hotel stay.

The LanChile conversions assume that 1 mile = 1.5 kilometers.