What is significant about shaking dust from your feet? If a house or town does not receive the Gospel, you are to shake dust off of your feet:


  Matthew 10:12
  12 As you enter a house, wish it peace.
  13 If the house is worthy, let your peace come upon it; if not, let your peace return to you.
  14 Whoever will not receive you or listen to your words—go outside that house or town and shake the dust from your feet.
  
  Luke 10:10-11
  10 Whatever town you enter and they do not receive you, go out into the streets and say,
  11 ‘The dust of your town that clings to our feet, even that we shake off against you.’ Yet know this: the kingdom of God is at hand.


I understand that the roads were not paved, and that people would travel by walking, so their feet would become dirty and dusty. I also understand that this is no longer a practice in some churches, but is in others, and that this was a practice among the Jews before Christ came.

But why? And why dust in particular? Does it have to be dust from that town? Is this act symbolic, or does the dust itself have some sort of effect if not shaken off? If it is symbolic, is it supposed to communicate something to the townspeople (so you would have to do it in front of them), or should it be done in the view of God only, or both?

What is significant about shaking dust from your feet?
In short, shaking the dust of your feet is way of indicating uncleanliness.
From the Believers Bible Commentary:

R. Guelich suggests that shaking off dust from the feet, the symbolic act that Jesus’ disciples were to carry out when leaving a rejecting village (6:11), has a threefold meaning (322–3):
•  It announces the inevitability of judgment (since further contact and opportunity for repentance is symbolically cut off).
•  It declares that the missionaries have done their job and are washing their hands of further responsibility (Ezek. 3:21; 33:1–9).
•  It labels that village as pagan.
Of these three, the third is the most provocative. Jews would shake off “pagan” dust before entering the “holy land” (Str.-B, 1:115). Likewise, Jesus’ emissaries demonstrate that villages rejecting them and the one who sent them have lost their inheritance in the people of God.
A similar message is conveyed via John’s symbolic act of baptizing Jews (1:5), an act normally reserved for converts to Judaism. John is thereby symbolically reinstating Jews into the people of God, implying that without his baptism, they are outsiders to Israel. The present text about shaking off “pagan dust,” however, takes this a step further. People are invited to reinstate their membership in the people of God; those who reject God’s messengers are symbolically excluded.
Other NT references to shaking off dust include all three of the meanings suggested above, though sometimes one of them is more prominent than the others (Matt. 10:14; Luke 9:5; 10:11; Acts 13:51; 18:6). Acts 18:6 in particular seems to highlight all three aspects. As he leaves the Corinthian synagogue, Paul shakes the dust off his clothing and says, “Your blood be on your own heads [meaning 1]. I am clear of my responsibility [meaning 2]. From now on I will go to the Gentiles [meaning 3]” (NIV). With his final line, Paul reverses the usual Jewish practice of cleansing their feet before moving from Gentile to Jewish territory. Paul is moving from Jewish territory (the synagogue) to Gentile territory (the house of Titius Justus) and shaking off the dust before doing so. Believing Jews join him as he moves toward a more fruitful mission field.

And from &quot;The New Manners and Customs of the Bible:&quot;

For Jews to shake dust off their feet was a sign that Gentile territory was unclean. In the New Testament this action indicates that those who have rejected the gospel have made themselves as Gentiles and must face the judgment of God. (See also Acts 13:51) To sprinkle dust on the head was a sign of mourning (Joshua 7:6), and to sit in dust denotes extreme affliction (Isaiah 47:1). “Dust” is used to denote the grave (Job 7:21). To lick the dust is a sign of abject submission (Psalms 72:9); and to throw dust at someone is a sign of abhorrence (2 Samuel 16:13; Acts 22:23). To bite the dust is to suffer a defeat. It became a common expression through its use in American movies about the early west.

In particular, I find the linkage between &quot;shaking the dust off your feet&quot; and &quot;biting the dust&quot; to be interesting.