Since stumbling onto this thread, I've done a bit of digging- from what I gather, plants absorb more ammonium than they do ammonia. Depending on the pH will depend on how much ammonia will convert into ammonium or vise versa- If the pH is low, the ammonia molecules convert into ammonium ions. If the pH is high, then the ammonium will turn into ammonia. The reason why this is is because there are more hydrogen molecules floating around in low pH than there is in high pH. So in low pH, the ammonia molecule picks up the extra hydrogen molecule, turning it into NH4+ (ammonium).
Also, if you keep in mind that the components that make up ammonia, nitrogen and hydrogen (also known as NH3), the plants can absorb the nitrogen part of the molecule, but don't absorb the hydrogen part of it. I suppose that being said, then the pH gets lowered, as there are more hydrogen molecules floating around and this will help more ammonia turn into ammonium, and therefore, become food for the plants! (that last part is a guestimation, of course ^.^; but it seems like it makes sense)
Plants are able to absorb ammonia, but it can inhibit growth- higher concentrations can kill plants. So long as the ammonia levels don't reach to a high level, then the plant is able to take it and convert it on its own >.> That's all I got so far, hope that helps you in some way of understanding how plants take in ammonia, or how they inadvertently absorb it xD