To call 911 for any purpose other than to report an emergency could result in criminal penalties. Each state has different penalties for 911 misuse, but in most cases, abuse can lead to jail time and stiff fines.
In California, calling 911 with the intent to annoy or harass an individual (such as calling 911 claiming a neighbor's house is on fire when it isn't) may lead to fines of up to $1,000, six months in jail, or both. To repeatedly call 911 in California for non-emergency reasons can lead to fines as high as $200 per call.
Virginia's penal code calls 911 abuse a "class 1 misdemeanor," which is punishable by up to a year in jail, a $2,500 fine, or both. In fact, in Virginia it's illegal to make false reports of emergencies or disasters by telephone to anyone - public or private - not just the 911 lines.
Emergency call centers are careful not to discourage proper use of 911, and most will educate callers that use the service incorrectly. Nearly all cases of 911 abuse that are prosecuted stem from egregious violations. For example, a woman in Oregon was arrested for calling 911 in order to see a particular deputy sheriff she thought was cute. Similarly, a Texas man called 911 to ask a dispatcher on a date. And a Florida man was arrested in 2003 after calling 911 more than 900 times.
If you are unsure when to call 911, then use your best judgment. It's much worse to not call 911 when a life is in danger than it is to call for less than an emergency.
Think in terms of immediacy: Do seconds count? If an intruder is in the house, the police need to respond immediately before someone gets hurt - call 911. However, finding a smashed car window and a missing stereo in your driveway in the morning can be reported on a non-emergency line.