The warnings are in every guidebook and it is a common topic among visitors as they sink into the sofas at a pavement cafe.
I normally treat the warnings of daylight theft casually, but I, also, felt that you could be at greater risk of being mugged or attacked in Phnom Penh than in almost any other city I have visited in recent years. I can't quite put my finger on why this is, but I did feel anxious when out and about in the evening. It really does not feel the place for wandering around with your camera round your neck or with an expensive watch.
I am sure old Cambodia hands will sniff and say "It's much better than it used to be" but that's no consolation if you become one of the statistics. Listen to what your hotel staff or restaurant staff say and be cautious.
Fortunately I don't speak from experience, but the first day we arrived in PP, we heard several stories. People walking with their camera on their shoulder, a few seconds later: boys driving by on their scooter, and byebye camera!
So, it's best to keep everything in your side, and wear it in front of you.
Crime is a bit higher in Phnom Penh than in other nearby countries due to the poverty of the people and the corruption of the government. While in Phnom Penh, someone went into my bag and stole $100 when I left it in my locked hotel room, and I got ripped off for much smaller amounts a few other times while dealing with locals. Also, it's harder to just relax and fit in here as a foreigner than in surrounding countries. In the local's eyes, foreigners = money, so we constantly get harassed and accosted while walking down the street. Squads of motorbike drivers sit on corners and harass you as you walk by - Moto-bike? You want moto-bike? Where you go? - even if you make it clear that you intend to walk to your destination. Sometimes when you're trying to cross the street a motorbike driver will stop right in front of you and make you walk around. I don't know what this tactic ever accomplished for them, besides annoying their potential customers.
Over the last 2 years the general safety has improved considerably and tourists are increasing in numbers. But take care not to be out after dark in Phnom Penh alone. Robbery at gunpoint is becoming more common.