I recently shot a wedding and some of my footage is peppered with little green squares which dance all over the frame. The sound is choppy and broken as well. The footage is useless and ruined!!

Are they "dropouts"? What is the cause? Here were the conditions:

1) It was a rainy humid day.
2) I used sealed, Fuji tape stock that I had for over a year.
3) I had not exclusively used Fuji in my XL1s in the past. I had used other tape brands as well (yes, I mixed tape brands before I knew it was so verboten).
4) I have about 20 hours on the heads.

HELP! What caused the picture deterioration?

It sounds like you have aliasing and/or dropout issues. They are most liekly caused by dirty heads. If that is the case run a cleaning cassette for 5 seconds and then record a test tape to see if it still has same problem. Is it possible that the footage recorded fine, and you are experience the problem only on playback? Do you have another deck to try it out on?

I have experienced the above issue during my video editing sessions. Which means, at least to me, that I captured the awful footage from the original tape.

You could assume it is the source footage that's run amuck if you didn't capture with the XL1S. If you did use the XL1S as a deck, there is a slim chance that the aliasing/dropouts are from the cam only.