It's definitely an authentication issue. I've used a Fedora 9 install and Diablo browsers to do "side by side" comparisons of smb:/ mounts. Using Konquerer I get a pop-up for "authentication needed" volumes, which I can then provide credentials. Neither the file browser nor microb provide such a pop-up -- and you're locked into things available to "public".

I'd /strongly/ prefer to not open up "public" to my home directory, and have hacked a bit at /etc/samba/smb.conf a bit -- but without any file-sharing goodness. (In particular; even setting the smb.conf file to my proper workgroup results in loss of SMB visibility at all!). Fairly broken model -- and libsmbclient /does/ provide such hooks. Too bad that the file manager doesn't cope with authentication at all....

In general, even the Canola PNP model hasn't been great in getting access to my music and video volumes (regardless of various agents I've tried) -- and, frankly, I'd prefer something beyond just PNP support -- the constant downloading of file lists is a bit painful if you've got a large-ish collection ....