Well, my OOBE (Out Of Box Experience) was terrific. (New computers just smell wonderful; better than new-car-smell, IMO.) Just pulled the thing out, plugged it in, and voila, up it came. Walked through the startup menus, and it just worked.
Well, right up to the point where I tried to connect to the network. It's upstairs, and my Linksys wireless router is downstairs, and to keep a very long story from getting out of hand, suffice it to say that I went up and down those stairs enough to get my legs sore, without getting the Mac to connect to the router.
I then proceeded to talk to two people at Apple and two more at Linksys, all four of whom said "we don't troubleshoot the other guy's products", (i.e. "the buck doesn't even slow down here").
So I went to the office store and bought a 50-foot cable, ran it through an air vent down to the basement, plugged it in, and it worked first time.
Bottom line, I blame Linksys.
Here are some more details; if you disagree, or have better ideas, I'd love to hear from you: Internet Connect sees the router, displays the WAP name, and shows a full strength signal; Network Settings can even get an IP address from DHCP. Elsewhere in Network Settings, everything else (like proxies, PPPoE, etc.) is turned off. As I mentioned, plugged straight into my DSL modem, or plugged into the router via wire, the iMac works just dandy. But when I plug the IP address of the router into Safari, it finds nothing, and says "You are not connected to the Internet", giving me a button to run the Network Troubleshooter. And I did upgrade the router's firmware to the latest, to no avail.
Oh well, we've got the cable run now, so all's well that ends well, I suppose.