Footwashing is a very old-fashioned church tradition, and I've never heard of it as a part of a wedding. I'd think it'd be hard to pull it off if the bride's dress includes a train.
Google Maundy Thursday for a full explaination, but here's what I can tell you:
In "Bible days", it was considered proper for a host to wash his guests' feet when they entered his house. After all, they lived in what we today call the Middle East, so walking from place to place made their feet dusty. They all wore sandals. People would've kept appropriate washing vessels and a stool near the door. Wealthy people added oils to the water for footwashing, which would've been very welcome in such a dry climate. Washing a person's feet was a sign of friendship and welcome.
However, the job of washing guests' feet fell to the lowest servant in the house. It wasn't something that the master of the house was expected to do himself.
At the Last Supper, only days before His death, when Jesus took his disciples into the Upper Room and they were together for the last time, He Himself bent down on His knees and washed the feet of every disciple.
Peter -- the disciple who often put his foot into his mouth, the disciple to whom many of us can relate -- initially balked when Jesus Himself knelt before him, saying that the Lord shouldn't take such lowly tasks upon Himself. Jesus told him that if He didn't wash his feet, Peter would not "belong" to him. In typical Peter style, he begged Jesus to wash not only his feet but also his hands and his head -- meaning that he wanted every part of himself to belong to Jesus.
Jesus explained that He did this to show that He had come not to be a king in the traditional sense, but that He had come to serve mankind. Obviously, the foot-washing was symbolic of His servanthood. You know, those who are first shall be last, and those who are last shall be first. What you do unto the least of My people, you do unto Me. The meek shall inherit the earth, etc.
When He was done with the foot washing, He urged them to think of themselves as servants to one another (and to the wider world) and to wash one another's feet -- again, a job that belonged to the lowest servant -- as a sign that they were to serve others. It meant that they were not to think of themselves as special and privledged because they'd been the chosen twelve; rather, they were not to hesitate to take any humble job upon themselves -- helping the poor, going among the sick, doing any menial labor that presented itself.
Today some churches hold a Maundy Thursday service the evening before Good Friday. At these services church members literally wash one another's feet as a symbol of their willingness to serve others.
Churches that consider themselves "progressive" tend to look down on this practice as old-fashioned. For example, in the novel To Kill a Mockingbird, one character is insulted as he's described as a "foot-washin' Baptist".
Personally, foot washing isn't necessary in the same way it was back in "Bible days" because we don't walk long distances through the desert, but I think our society would be a whole lot better if more of us DID think of ourselves as servants to one another, and the world'd be a better place if we didn't all have so much pride.
