I think the Gameboy Player is pretty much a guarantee that Nintendo doesn't plan on using the existing Broadband adapter for their "LAN Plan". It would just be far too much of a hassle for those who use their modems to play games like Phantasy Star to unplug the entire setup just to switch to their broadband adapter for LAN plan. Plus, that's extra hardware sales down the drain. I think it's more likely that Nintendo will opt to use the one remaining expansion port for whatever LAN accessory they have up their collective sleeve.
I don't doubt that console LAN play is fun, don't get me wrong. I use my PC for LAN games all the time. It's a blast. I've even joined in the event of ultimate "nerd-dom" on occasion - that's right, a computer LAN party! The question is whether or not the whole endeavor is worth their time and money. If Nintendo honestly thinks that online isn't worth investing into because of lack of consumer interest, thereby making it unprofitable, then I'd really like for them to explain the logic behind a LAN setup. I still believe that it will be the minority that takes advantage of this, though it seems that most hard-core Nintendo fans (IE, the ones that post on Nintendo news site message boards) plan to at least look into the idea.
I am intrigued by noentiendo64's comment, though - that Nintendo is slowly moving towards a hybrid portable/home console. This would certainly be a step in that direction. Same goes for their revealing of the high quality monitor and small discs. Indeed, it seems they've been trying to tie the Gameboy and the Gamecube together this generation, as is evident by their connection device endeavor. Even the names, Gameboy and Gamecube, are very closely related, as if Nintendo is trying to blend the lines between them a bit in preparation. Imagine a device with a flip up monitor that you could take on the road with you, but you could also inherently plug into the TV in the comfort of your living room, all with comparable graphics and gameplay to Sony's and Microsoft's latest. Perhaps you're onto something...
Bloodworth: Do you have any more details on Nintendo's online experiments in the past? I'm not familiar with them.
Grey Ninja: The XBand wasn't a Nintendo product, though I forget who the mastermind behind it was. But I remember seeing one for the Sega Genesis as well, so there's no way Nintendo had any part. As I recall, it was very similar to what Microsoft has set up for the XboX Live - a proprietary client-server model with fees, as AngusPodgorny put it.
-Matt