I think it really depends on how much bandwidth Nintendo plans to use for the online plans. If they plan to have thousands of people playing high-bandwidth games at the same time, well, that costs a lot of money. And there's certainly no profit in providing it for free.
Most of my complaints with monthly fees for gaming is that the fees are rather high. $9 a month seems low, but after playing online for 6 months (with the first month free), you've bought the game again. If it adds a ton of replay value and fun, then that might be justifiable, but if I want to play 5 or 6 different games, all with monthly fees? That really adds up fast, and the more games you play, the less time you can spend with each one.
I think if Nintendo offers a low monthly fee that's payable in different amounts (monthly, bi-annually, annually), it'll be do-able. Of course, I think they'd have to have some free months included in order to combat the Xbox Live thing, if that's a factor, and something to say "hey, we only have a few games, so we're not charging you for them for a while." If they only have AC2 and Mario Kart online, then that's not a lot of bandwidth and they could host their own servers without a lot of overhead costs, and possibly introduce a small monthly payment for Nintendo Online.
Or, since they seem pretty keen on letting people do their own thing (4 player games out the wazoo), they might simply do a peer2peer network model, which would bypass the need for much bandwidth. All they'd have to provide is the online meeting place for people to go and find games. That just needs chat capabilities (if they ever release a keyboard), or simply a list of available games and the info about those games. They might even release devkits that allow the game developers to set up one cube as a dedicated game server for such games, for people who have 2 cubes or a really fast connection.
Does Xbox live actually test the connection speed? Cos you could get online with 56k with it relatively easy -- just connect it to a computer and bridge the connection over to the modem. hmm...