Yes, I am all-but-certain that is the heart of the problem. I was originally going to go with the subdomain structure, and removed the www from some of the settings in preparation for that...then decided not to do it afterall (see what you get for being wishy-washy) :)
I think that, once you enable the network, the blog URL fields disappear from the settings panel. Kind of confusing, to see them, and then not see them (I'm scratching my head saying "am I going crazy?")
Ultimately, I logged into the database and fixed the setting, removing the www from the blog address. That allowed me to log in as network admin.
But my Firefox refuses to accept that answer, as it keeps trying to "over-guess" my inputs, replacing them with the "www" no matter what I do (clear cache, drop cookies, etc). When I use Safari, it's all good.
Now that I have had the chance to use it a wee-little-bit, the Multisite functionality seems to have some inefficiencies for the writer, IMHO. I don't have the PHP chops to do it differently, but I can see it in my head as making posting to multiple blogs much more convenient.
For instance, if you are the super-admin, and have one or more blogs, why doesn't the post panel just give you a "master control" where you could check a box (as you would for category) for a given blog, and it posts the content to the checked blog. As well, if you wanted to post the same story to multiple blogs in your network, tick the checkboxes for the blogs you want that story posted to. Is this already possible, and my install is gummed up, or is there some stylistic reason why it's not done this way? What am I missing here, because it seems so "slam-dunk obvious" that the smart folks who developed this functionality surely would have thought about this.
Nonetheless, very grateful Multisite exists.
Thanks for your reply, Ipstenu, and your eagle eyes on the URL string!