I wouldn't do this unless you plan on potentially not having this friend at all later. I have fallen in love with three straight guys. The first two I told and our friendships did not last much longer. Not because they were homophobes, but because I obviously was not interested or content with just friendship anymore after telling them and it led to lots of awkwardness.
The other friend I fell in love with, I did not ever tell. We are still great friends to this day and I kind of have feelings for him, but not so much now that I have a boyfriend I am in love with. I'm really glad I never told this friend because it would have made our friendship awkward. I understand when people say, "If they are real friends, they won't care," but seriously, knowing someone is in love with you - regardless of their gender - is going to affect how you treat them/they treat you. I'm 100% gay. If one of my best girl friends told me she was in love with me, I would definitely be okay with that, but I would definitely not treat her the same. If I had been touchy-feely before, I would be much more hesitant. I would try not to get too close so as not to lead her on. I would wish that she just hadn't told me, nice as it is to know someone is in love with you.
Only way you tell a straight friend you are in love with them is if you are positive they are actually not straight and probably feel the same way as you. But in that case, you two probably would've hooked up by now anyway.
I'm just speaking from experience...
