During the last couple of years, safety in Cambodia has improved a lot. The main problem in rural area are still the landmines (especially near the border to Thailand).
The Khmer Rouge have been dissolved in 1998. So the threat of being hijacked has vanished. More of a problem nowadays is crime, which is on the fast rise. Especially Phnom Penh is getting more and more dangerous. After dark, robbery at gunpoint seems quite common. There are several reports of tourists being dragged from their motorbikes. Make sure that you don't stay out in the evening.
Be careful venturing out in the countryside by yourself. Stay to tracks recommended. Landmines are still a very big concern in Cambodia. You see many amputees - young and old alike in the country.