Well this year will be my 14th Festival, so I guess I'm a bit of a veteran. It's a tough one to answer, because lots have things have been improved over the years - drainage, concreting the roads etc. But I don't think moderate rain leading up to the festival will be a major factor in causing a mudbath. I think with it being so dry up to now, the rain will just run off the fields and into the drainage system.
It doesn't take much to dry the site out at all. The soil is mainly clay, so water tends to rest on it rather than soak into it. This only becomes a problem if there have been weeks and weeks of rainfall, or there is the factor of hundreds and thousands of people putting pressure on the water, pushing it into the ground and then churning it up as thick, sticky mud. Add to this further rainfall, and you get the lakes of mud some of us are unfortunate enough to be familiar with.
That's my opinion chucked in, others may differ?