Given the performance advantages of plasmas over lcd's, would this then be negated by placement in a bedroom against a wall which is accross the window?
Yes, it will. The bright light from across the screen will cause a dimmer picture with washed-out colors on a plasma.
Would the reflectiveness issue be such that I would be better served getting an LCD as opposed to a Plasma in this kind of set-up?
Yes, an LCD would be better.
But I wouldn't worry about relfections if I were you. Notice that the highest-end LCDs these days are even more reflective than the high-end plasmas.
The primary factor to consider is the brightness of the ambient light. Reflectivity is only secondary.
There's just no way around it. If you want a great picture, get a dark room and a plasma. If you're OK with less than great, a bright room and an LCD will do. The kitchen TV should be an LCD. The TV in the dedicated video room should be a plasma.
But don't take this as gospel truth. The vast majority will prefer LCD no matter what anyone else says. And that's fine with me. LCD owners love their LCDs; plasma owners love their plasmas. Everybody happy.