...the real issue, though, is that while WiFi or a USB-to-Ethernet dongle are fine for web browsing and email (and are probably faster than your broadband) they are simply no replacement for a gigabit Ethernet port. If you want to access large files over a local network they just don't cut the mustard (1Gbps from Ethernet vs. 300Mbps from Wireless-N with pigs on the wing and tap-dancing mermaids, or 480Mbps from USB 2 downhill with the wind behind it and the gods smiling).
USB 3.0 (which is predicted to be on their new line) makes gigabit USB ethernet feasible and inexpensive (at least in theory). Apple could even throw the adapter in the box, just like they do with the HDMI -> DVI adapter in the Mac Mini's box.
Moreover, how much time do people actually spend copying (as opposed to accessing) huge files on their local network? Some of my video files are up to 40 GB in size, but I don't spend time copying them to and fro. When I watch them, they stream just fine over wireless N. After all, I don't need the last bit of a 2-hour movie on my computer until 1:59 after I started watching it.