(updated 16 Aug 07) 

...I'll scratch yours.
(Suggested by NITRAMXXX@aol.com)

If my lottery ticket isn't a winner...I'll scratch yours. (rockitower@aol.com; rod.renner@juno.com; hundreds others)

You mess up my chalkboard...I'll scratch yours. (GrigsbyOK@hotmail.com)

Honey, I'm a little tired. Do you think you could do the dishes, and hang up the laundry, and feed the cat - wet food not dry - and take out the garbage, and could you rub my feet - that would be just heaven - and scratch my back - I've got a real itch - and if I manage to find the time...I'll scratch yours. (tpanner@hotmail.com)

You lick my taint...I'll scratch yours. (TheEyeWit@yahoo.com) Something tells me t'ain't going away any time soon.

I've got the winning horse in the derby race so,...I'll scratch yours. (Airfarcewon@aol.com; lilfishjean@sbcglobal.net)

You sniff mine, and...I'll scratch yours. (n_zukowski@hotmail.com; Mistahtom@aol.com; and many, many more who liked to be sniffed)

If you feed the squirrels nuts...I'll scratch yours. (sunni61773@aol.com) Hmmmm...

Give me a ten foot pole and...I'll scratch yours. (TheEyeWit@yahoo.com) I always thought it would have to be an 11 foot one.

There's no way in hell...I'll scratch yours. (scalpel@aol.com)

"It's still 200 bucks an hour honey, if that's what you want...I'll scratch yours." (DavidGoTribe@aol.com; joseph-blevins@sbcglobal.net) You both said $200 - I guess that's the going rate and all...but you'll be the ones trying to explain your way out of this one, not me.

If you scratch that scaly, unreachable patch of skin on my back...I'll scratch yours. (tpanner@hotmail.com)

If you get one little scratch on my car...I'll scratch yours. (tpanner@hotmail.com; joseph-blevins@sbcglobal.net)

If you beg me, then...I'll scratch yours. (ReineDeDouleur@yahoo.com) And, honestly, isn't this the way it should be?

I married you, so I guess...I'll scratch yours. (DaphnetheRed@yahoo.com) Ahhhh...so soon the honeymoon's over.

As the say in billiards: you scratch my balls and...I'll scratch yours. (maxcel200@aol.com)

Oh-so been there...done (or should I say "didn't) do that...

I know where you've been. So, you're crazy if you think...I'll scratch yours. (GerriHan65@aol.com)