I generally like cardio before resistance training 'cause it warms me up... but I am a minority in amny cases. But I have found it gets me alert, warmed up and primed to get it on...
Many feel cardio before will effect strength output - and most cases it can, especially for your legs - kind of a prefatigue for them... but this is also depending on how intense and how much resistance was used during the cardiovascular work...
If the cardio is really intense (brings your heart rate up high) you can burn up your immediate energy stores before moving into resistance work - then your body will be forced to produce more energy from a less immediately available sources (unlike the stored glycogen - which was readily able to be used). If the cardio is pretty moderate - you can tap those immediate stores but not completely drain them... the more intense it is, the more the body will use what burns the fastest.
Logs (like stored fat) will burn long and hot - but tissue paper (like stored glycogen) burns quickly 
Keep the heart rate down to where you can still talk comfortably and your body will be more likely to use fat for fuel... effectively warming you up without completely draining you...
I generally have liked to get warmed up for heavy weights with some cardio and then after the resistance training move right into post-workout nutrition...
All this does depend largely on goals and nutrition... as well as your somotype and general metabolism...