Wow, I'm surprised you guys don't know. They have a sarcasm mark, very cleverly named the SarcMark
They tossed around the idea of an upside down exclamation point, but ended up with the curly Q.
I'm also surprised Magister Ventrue hasn't created his own version of this, and added it to his sites.
Since I posted, I'll add...
To Reverend Bill_M,
This post is very funny.
I love the red Devil Ducky, on your Y!A profile.
I'd say more, but when it comes to you Bill_M, it's hard to improve on perfection.
Have fun writing with Sarcasm, the Internet can understand.
Hail Satin (Sheets and rubber Devil Ducks)