how can antifreeze get into your intake

Blown head gasket, cracked head, cracked block. These are a few things that could be the problem.

Gasket from the heads is where you start. A good mechanic can do a few tests to tell if it is a gasket or cracked head or warped head. A 12 year old car without knowing the past history, could have several items that may go wrong as stated from BoholMark. Did the engine overheat at all? If so it may of started as a bad gasket, due to a warped intake manifold, then the aluminum heads or engine block may have cracked or warped due to the excessive heat. Best to worse case.