It was Jewish tradition to wash one's feet before reclining at a table for a meal. As this was normally done by a servant or slave, and since Jesus didn't have any, Kostenberger presumes that this hadn't been done. Nevertheless, it was a cultural norm; it needed to be done and it was a surprise that it hadn't been done already.
I believe this is a testament to something important relating to Jesus and his disciples. The fact that Jesus and the disciples had already reclined at the table to eat without accounting for the cultural norm of footwashing suggests that Jesus was more informal with the disciples than many teachers would have been in his day. He was more of a mentor to them than a rabbi.
Jesus teaches us that everyone in his Kingdom is a servant, including the Son of God himself. If he has left us this precedent, then there is no excuse for anyone not to comply with his ministry. This was, as Kostenberger points out, a horribly convicting moment, causing the usually impetuous Peter to flip back and forth between extremes. In fact, his response is indicative of what the disciples all undoubtedly felt. Pushed against a rock and a hard place, with old culture on one side and Christ's call on the other, they had no choice but to make a decision.
Kostenberger makes it clear that the church was never supposed to implement actual footwashing as a rite as that would have been to institutionalize what Jesus meant to be an example. Therefore, the key is to know that it was an example. Finding examples in modern life are not difficult. Outreach magazine reported on a church that placed urinal cakes with the name of the church on it in local public restrooms as an example (no citation because I can't find it in my stack of past copies). I don't believe this was a good example, but it does show that churches are at least looking for them. What would have made a good example would be adopting a public restroom and cleaning it. Caring for the hygiene of the homeless would be another example, as I saw personally with The Outpost Ministry in San Francisco. Finally, I think that (inside the church) cleaning a fellow believer's house or clearing out a garage or gutters might be a more precise example, as Jesus was ministering to believers in the actual example.
 Andreas J. Kostenberger, Encountering John: The Gospel in Historical, Literary, and Theological Perspective (Grand Rapids, MI: Baker Academic, 1999), 146.
 Ibid., 145.
 Ibid., 147.