I've got a spray computer. It monitors ambient temp and I/C temp, as well as the duty cycle of the fuel injectors. When the injector duty is high, and the temp difference is great, it sprays the water.
The unit I got is from Australia, and I bought it used, so I don't know where you can get one from new.
Here's the article describing the system:
http://autospeed.drive.com.au/cms/A_0527/article.html