Well, I found the box with commentaries, so I've been taking a look at Fitzmyer's commentary on Luke again. I also checked out the commentary over at Working Preacher (www.workingpreacher.org).
Regarding the commission Jesus gives the seventy, Fitzmyer writes, "They are not being sent to carry out out ordinary social obligations and amenities, for what they are to preach and do will set them apart. There will be no time for ordinary greetings, scruples over what sort of food one eats, or searching for better quarters. . . Their concern for God's kingdom must be that of reapers confronted with a harvest that is to be gathered in before it spoils." He also notes that in Luke, the commission of the disciples and that of these seventy others form a doublet — in other words, Luke has doubled up a particular narrative, which is presented only once in the other synoptic gospels (Mark and Matthew). Luke is emphasizing that many will carry on the ministry of Jesus, not just the twelve disciples; this perspective becomes even more clear in Acts.
As for the report of the seventy in the second part of the reading, Fitzmyer emphasizes the final part of Jesus' response: "Jesus' last remark (v. 20) puts the mission of the disciples in its proper heavenly perspective. It is not just that Satanic evil has been eclipsed — this is not the reason for their joy — rather, the real reason for it is that God himself has inscribed the names of these representatives of Jesus in the book of life."
Over at Working Preacher, Dr. Salmon focused on what the experience of the seventy must have been like. She lifts up the importance of table-fellowship and hospitality in the gospel of Luke, noticing how this is played out in Jesus' instructions: "Remain in the same house, eating and drinking whatever they provide." She writes, "This model of hospitality transgresses common customs of hospitality as I know and understand them. Where is the notion of reciprocity? If I invite you to dinner, I will notice if I do not receive an invitation to dine at your home. And what about overstaying one's welcome? We have unflattering names for people who 'take advantage of' our generous hospitality. The name 'free-loader' comes to mind. . . When I heard his story, I pondered an underlying assumption that those who have more extend hospitality to those who have less." What Jesus is instructing is a different notion of hospitality than that with which we are familiar. If we put ourselves in the position of the seventy, as those carrying out Jesus' ministry, then we have to examine our assumptions about hospitality. Maybe this also speaks to the faulty notion of ministry as 'helping people' (in the sense of constantly giving, trying to fix, and assuming we are the 'haves' and others are the 'have-nots'). To be ministers in Jesus' work, we have to accept hospitality, stay in the home where we are welcomed, and eat what is set before us.