I saw that study on the news this morning. I find it interesting since all of the women on my mom's side of the family have gorgeous voices and flawless pitch. My identical twin sister and I on the other hand pale miserably by comparison.
I can sympathize because I too can hear tunes flawlessly in my head, but my voice just doesn't translate it the same way I "hear" it. I agree with the idea that singing practice can help. I'm surprising my sister by learning a bunch of Irish folk tunes to sing to her baby (she's due in the fall so I have plenty of time).
I find that I am having to listen to songs over and over and over and then sing a line over and over and over until I get it down. And even then I think I'm varying the tune ever so slightly. But that's what's great about Irish folk tunes. So many variations, nobody's going to be able to tell if I screw up a note here or there as long as it blends into the melody.
I also wonder if it's a confidence issue. I heard all these beautiful female voices around me growing up and always felt I could never sing that well. Maybe it was a self-fulfilling prophesy. I'm also learning these songs to sing them around the house in hopes that my sons will be encouraged to try out their voices if they get used to having self-made music around. Maybe one or both will be lucky enough to inherit perfect pitch from their grandma.