I can't beleive how many people are still overanalyzing this "Free Online" statement Nintendo made, including some of the major sites. It's all very simple.
The Online service will be like Xbox Live, except that it won't cost anything (and very likely, will not be nearly as well polished). The actually way you get online may or may not be free, depending. If you're at home, you'll need a broadband connection and a wireless router, and that's going to cost some money. If you're on the road, you'll need access to a free wireless hotspot. Paid hotspots might not work, because they usually require an HTML page to be displayed where you can enter a username and password, and I doubt each DS game will include a web browser to do that.
Seriously, this statement in no way means that Nintendo is rolling out some sort of nationwide network to bring wireless internet access to everyone. Occam's Razor was never more applicable than in this case ... the simplest answer is often the correct one.