iCal dock icon displaying wrong date.

Huh... if it were the 17th, I'd say you were getting the default icon (the default is July 17, which is of course iCal Day). In 10.4 or earlier, you'll get this any time the app isn't running, in fact. (You are on 10.5, right?)

But you're seeing the 18th, I'm assuming of this month, not July (turn on Dock Magnification if you can't see). 10.5 added a little feature where the app adds a little extra to launch through the Dock that updates its icon whether it's running or not. I'm guessing that this extra must have either frozen or otherwise glitched out, so that it's now stuck while, when the app is running, iCal takes over and makes it right.

Two things to try would be to remove the app from the Dock and re-add it, and if that doesn't help, reboot.

If neither of those help, try changing the date to something else manually--does the icon stay on the 18th, or does it update to one day before the new date?