I was looking at the purchase of the iPort a week or 2 ago, but didn't buy one.
The device has 'wings' that are tightened against the edges of the drywall. Fits into a cutout.
The 'wings' appear similar to what you'd find on an old-work junction box/LV bracket.
Click on the link for the IW-20 Series manual.
http://www.iportmusic.com/products/in-wall/IW-20
The iPort website has a tool that helps you choose which iPort you should get; make sure you have the right manual.
I don't know if the older models use the same method - I believe you can find manuals for older iPorts, too, if you're considering ebay.