I've always been able to sightread, even when I'd only just started playing instruments. As long as I knew the notes, I was okay. The only times I have trouble sightreading are when it's something right at the bottom or top of my range. It's actually fairly scary.
What's really annoying is in my choir, I'm the only person who can do it fairly well. The first time we sing a new song, everyone else is mumbling the words or singing really quitely, and I'm singing like I normally do.