Yeah the problem is a lack of soil structure to carry harvest equipment when it gets muddy.  Have a buddy who was plowing alot of his corn on corn acres until the wet fall of '09 when the lack of soil structure let the corn blow down and it was so muddy it was next to impossible to get the combine through the field.  If I remember right he had two 12 row combines and a 6 row and was struggling to get 30 acres a day combined.