Yes, fermentation releases heat into whatever vessel the wort is in - sometimes lots of heat. It can be as much as 4C. As to what temperature this starts, that is dependent on the strain of yeast. If you are on the lower end of the recommended range for the yeast, activity is slower and produces less heat. As you raise the temperature, the fermentation becomes more active and produces more heat, which in turn, raises the fermentation temperature even more. So yes, you have to account for the temperature increase in your ambient temperature. I try to keep my fermentations cool for the first 72 hours (peak activity) to make sure the temperature stays in control.
The heat increase at the peak of activity (and the off flavors it can produce) is why many homebrewers fight so hard for temperature control in fermentation. Some (including JZ) believe consistent, controled temperature during fermentation is the best way to improve the quality of your beer.