Put me down as a "not going to do it" mama. I didn't want to know for my first child, and since my DH, who wanted to know, can't keep his mouth shut, he didn't get to find out. In the end, he was really glad for the surprise.
This time, I'm leaning towards no ultrasound unless medically necessary, and definitely do not want to know (though I already suspect another DS!)
not sure yet. i found out with the other two - part of me wants to be suprised at the birth, as this is likely the last for us. OTOH, i already REALLY REALLY want to know, and dh is against waiting (and there's no way he could keep a secret from me!) - so, we'll probably find out.