No, there won't be an app for this.
If you're thrilled about Apple's iCloud announcement this week, don't drink to it yet. Apple has just updated its App Store Review Guidelines, in which it now explicitly bans the implementation of driving under the influence (DUI) checkpoints in apps for iOS-based devices.
Section 22.8 of the updated guidelines clearly states that "apps which contain DUI checkpoints that are not published by law enforcement agencies, or encourage and enable drunk driving, will be rejected."
This new development is likely Apple's response to the call of U.S. senators a few months ago, asking Apple, Google, and RIM to pull police-evasion apps from their mobile devices.
The new guidelines only apply to DUI checkpoints specifically; apps that pinpoint "speed traps," such as Trapster, are still available in the App Store. Note, however, that Apple only bans DUI checkpoints that are not published by law enforcement agencies, and in California, the law requires that the general locations of these checkpoints be made public in advance.
So far it seems Apple is the first to change its policies accordingly, while Google's and RIM's app review guidelines remain the same for now.
It will be interesting to see how this ban will affect users, as generally if you're still able to worry about DUI checkpoints or even to run an app, you should be coherent enough to know you should call a cab.