[EDIT] SIGH As expected, 2 minutes after I posted this, after a couple hours of waiting, everything seems to be working fine. Never mind. I'll leave my stupidity below for the sake of posterity. The solution is to simply be patient.
Every quote I've read and in this forum implies some understanding that I must be too dense to get. I want to see files in my new DH space, though I still have my site hosted elsewhere. I created a simple file:
I can't see that with http://subdomain.dreamhosters.com/index.htm.
I've also tried /home/myuser/mydomain.tld/htdocs/index.htm, www.subdomain.dreamhosters.com, changing permissions to 755, etc. Every attempt returns a 'cannot find server' or 'page cannot be displayed' type error. A couple hours from creation of the subdomain, I do see it in the panel under Manage Domains, and I also see the "non-editable DreamHost DNS records for subdomain.dreamhosters.com: nrnd, ftp.nrnd, and www.nrnd are A-types pointing to the right IP address.
Here enters more brain density...
What do you guys mean by "mirror"? I haven't yet flipped the switch for DNS to point to DH for my domain. How does sub.dreamhosters.com "mirror" www.mydomain.com? I believe this means the subdomain should simply point to the DH space allocated to this domain (yes, this is the one listed in Manager Domains) but the term mirror as used by some people seems to imply that files will be sucked out of my other host and that a mirror site will be created in the DH space. I know that's not what's intended but that's what a site mirror typically is and sometimes it sure sounds like that's what people here mean.
Outside of that bit of brain fartage I think the inability to access the subdomain is something outside of my control. Advice is well appreciated.
[Edit] Maybe this is a clue: ftp.sub.dreamhosters.com returns the following:
Site Temporarily Unavailable. We apologize for the inconvenience. Please contact the webmaster/ tech support immediately to have them rectify this. error id: "bad_httpd_conf"