Curses!

Give it your best (humorous) shot...

[tangor]

May the fleas of 1,000 camels infest the crotch of your enemy, and his arms be too short to scratch.

[weeks]

Isn't that a blessing of sorts, not a curse?

[LifeinAsia]

My favorites:
May you live in interesting times.
May you get exactly what you wished for.

[rocknbil]

. . . you, and everyone who **looks** like you.

[blend27]

May the EXTRA DOT be found in you enemy`s COBOL Source code in the wrong spot on Sunday.

[WesleyC]

Your compiler's so fat, it makes the Hindenburg look like a microbe.

[MatthewHSE]

There's the ever-popular parent line:

"I hope you have a child just like you when you grow up!"

Never said to me - although I'm sure it was thought on occasion!

[MatthewHSE]

A few more humorous tries:

"May your clients' ideas be precise and their descriptions hazy.
"May your bicycle pitch you forth to land your ribs on frozen mud."
"May the pollen of 1,000 ragweeds lodge in your nostrils."
"May your hard-drive seize and your backups be corrupt."
"May your in-laws be many and vocal."
"May an old egg and a fresh skunk be your constant companions."
"May your nights be short and your dreams be of code."
"May you never lack a boss and a deadline."

[rocknbil]

"You're an inspiration for birth control." -DukeNukem

[bhonda]

"May your in-laws be many and vocal."

Yes. Brilliant.

[tangor]

If at first you don't succeed, just be Cuil.

[Poppy122]

May you become famous when they name a disease after you.

[Essex_boy]

I hope you have a child like Essexboy....

[tangor]

May your website become MFA and get a dozen DMCAs!

[tonynoriega]

May your fingers be too fat to type code.

[Edwin]

May you not live to regret it.

[Anyango]

May Every Murphy Law Apply On You :)

[wyweb]

I was always a big fan of a simple, "yo mama."

[Matthew1980]

I heard this the other day, from a father to his 14 year old son:-

"Do that again, and I'll go back to dial-up"

The lads expression was priceless >:{

[C7Mike]

May your son marry his anime pillow.