I think that the lines can and will get blurred!!!
I speak from experience and from watching one of my girlfriends go through the same thing.
Once you get comfortable with the ex, you know, feelings start to stir and you remember all the good times and downplay some of the reasons you both broke up and before you know it..WHAM!!! yall doing the do

ha ha
anway, I think that if you and your ex have a good understanding of your new relationship, and you both know where you stood, then being friends shouldn't be a bad thing, as long as you don't make things more then what they are.