I heard local radio guy Corby Davidson, who lives in the Lake Highlands area, talking about this on his show yesterday (and tweeted about it): his sprinklers were shooting water every which way, flooding the street, and he assumed they were broken.
Turns out more than a few sprinkler valves in his neighborhood had gone missing. A quick search reveals that this is not a totally unique case. Thieves sell the valves for the brass inside, which is worth about $2.50 a pound as scrap metal, according to one article.
In fact, the issue is common enough that the online sprinkler warehouse sells theft prevention cages for sprinklers.