|MadSci Network: Chemistry|
The explanation for this is the same as that for CO2 dissolving in water.
This is a very good reference on that.
The first thing to remember is that water is NOT ionic. It is a polar covalent compound. SO2 and SO3 have polar bonds also, so they and water are more alike than you think. The second point is that of equilibriums and thermo. The reaction of SO3 is very exothermic (very favorable) and not reversible. Also note, sulfur trioxide is actually a liquid allowing easier dissolution.
Now if you are studying formation of acid rain, that is the reaction of SO2 with water in the gas state of waer, not liquid.
Try the links in the MadSci Library for more information on Chemistry.