Exact numbers are impossible to come up with for one simple reason - US doesn't report the ASM's operated by contract carriers. Given that, here's some comparisons.
3Q05 vs 3Q06 Mainline ASM's (3Q05 was the last pre-merger quarter)
3Q05 US ASM's - 13.225 Billion
3Q05 HP ASM's - 07.875 Billion
3Q05 Total ASM's - 21.100 Billion
3Q06 US ASM's - 12.589 Billion
3Q06 HP ASM's - 07.567 Billion
3Q06 Total ASM's - 20.156 Billion (-4%)
For a closer look, here's the monthly mainline ASM's beginning with Sep 05 (the last month before the merger):
Monthly Mainline ASM's
09/05 - 6,415,467,000
10/05 - 6,476,882,000
11/05 - 6,097,845,000
12/05 - 6,288,837,000
01/06 - 6,193,686,000
02/06 - 5,501,570,000
03/06 - 6,534,770,000
04/06 - 6,304,584,000
05/06 - 6,625,307,000
06/06 - 6,704,703,000
07/06 - 6,966,463,000
08/06 - 6,890,254,000
09/06 - 6,299,102,000
As you can see, in only one month did ASM's drop 10% below the Sep 05 number - Feb 06 - and the next month they were above "pre-merger" levels. I believe HP was running as many as 5 paint lines around that time, and obviously winter is the maintenance period for the US widebodies, so does a one month drop constitute a "cut in capacity"?
Finally, the combined DL/US ASM's for December - 18,873,850,000 (including all DL express operators but not including US contract Express carriers). 10% of that would represent about 1/3 of all reported US Group capacity (6,752,129,000 ASM's), about 50% of US (East) mainline capacity (3,977,206,000 ASM's), or about 75% of HP mainline capacity (2,477,524 ASM's).
Jim
[Edit to add]
I forgot to add this for those that say most of the reduction will come from express. All the reported express capacity - DL's owned/contract plus US wholy-owned - doesn't equal 10% of the combined capacity mentioned above. Even "guesstimating" US contract express capacity, does anyone really think most of express will go away?