Thanks again Mika.
Unfortunately, as I noted in my initial post, I am using subdomains.
Also, as I indicated in that initial post, I'd tried the Sidebar Login plugin already (I'll have to backtrack to see what did/didn't work for me with that one).
I'll also check out the what you posted in #2 for me -- much appreciated :) -- but do you know of a way to get things to work for how I need them to work (one login allowing login/commenting on all sites in the network)?