Thats on thousand fist (or thunder fist), for simplicity ill explain this assuming your using a 2 hander.

Its easier to look at it on ashen hand to start.
You quickly hit for 3200 additional dmg.
You ashen hand target for (3200+1600)


Since thousand fist is a 3 hit attack
You strike quickly for 2000
You strike quickly for 2019
You strike quickly for 1991
Your thousand fist hits for 12,000 ( 6,100+5,938) 

The numbers in ( ) are very close to original dmg and dmg added)

Tested this last night using both 1handers and a 2hander....
When using 1handers i got (2400+1200)
When using 2hander i got nothing....only You hit uber_mob01 for 2400 damage. No (..... + .....) so this damage in () is mainhand + offhand damage when dualwielding and has nothing to do with the celerity damage added. Ergo there its very hard to see what celerity actually adds to your normal damage other than that blue text. Which again means its hard to know if it actually works at all. They did nerf it tho ....that im sure of....back in the days when i used it i hit easily for 4 x 30-50k damage in a normal group with a bard.....now i rarely go over 4 x25k damage raidbuffed and with bard.
Considering the damage a rogue deals it wouldnt hurt if they actually gave us:
You hit uber_mob01 for 20.000 damage
You quickly strike for an additional 20.000 damage
You quickly strike for an additional 20.000 damage
You quickly strike for an additional 20.000 damage
You deal a total of 80.000 damage to uber_mob01

Edit: When i say 4x30-50k and 4x25k i mean with thousand fists line (im drunken) as we get 4 hits in thousand fists...not 2.

I'm only lvl 38, but tested this a bit before and after GU4 and haven't really noticed any real difference in the dmg. The difference I've noticed is how it is displayed in the combat log.

Pre GU4 it displyed your total dmg all hits.

Post GU4 it displays each true hit of TF then the total dmg at the end.

Like Fujitsu said the easiest way to look at it is using AH. But to look at it using TF you would have to break TF up into it's True attacks. Because TF is really 3 attacks (for me atm) Not one, like it displays. So in it's true form it would read something like this:

You quickly strike for 1000 damage
You Thousand Fists hit for 2000 damage (Total dmg: FoC + TF)
You quickly strike for 1000 damage
You Thousand Fists hit for 2000 damage (Total dmg: FoC + TF)
You quickly strike for 1000 damage
You Thousand Fists hit for 2000 damage (Total dmg: FoC + TF)

For a Total dmg of 6k.

But this is really spammy so instead they display the FoC hits (3 because TF is 3 hits) and the total dmg:

You quickly strike for 1000 damage
You quickly strike for 1000 damage
You quickly strike for 1000 damage
You Thousand Fists hit for 6000 damage (Total dmg: FoC + TF)

But I do agree it's hard to tell what your FoC is really adding, I just normally 1/2 what my TF does and that is normally pretty close.

But I wouldn't mind seeing it as:

You quickly strike for 3000 damage
You Thousand Fists hit for 6000 damage (Total dmg: FoC + TF)
or
You quickly strike for 3000 damage
You Thousand Fists hit for 6000 damage (3000 + 3000) (Total dmg: FoC + TF)

1 handers would be:
You quickly strike for 3000 damage
You Thousand Fists hit for 6000 damage (2000 + 1000 + 3000) (Total dmg: FoC + TF)

Something like that anway

The you quickly strike for X dmg is the EXACT amount of dmg bonus added to the attack.
 So do what zakkoris said, take the total dmg....subtract the quickly strike dmg... then thats your base dmg

You quickly strike for 3000
Your thousand fist hits for 6000

Hit for 3000, celerity added 3000, total dmg 6000