I ran into this recently.
Try resetting the crawled content:
technet.microsoft.com/en-us/library/cc262653(office.12).aspx

Make sure the crawl time is configured also.

but wow, those instructions were terrible (the page
names and sequence didn't make any sense at all; I checked three times just
to make sure it was intended for MOSS 2007). Thanks, TechNet!

I managed to reset the crawled content. Now when I start a Full Crawl, it
finishes in 2 seconds. No errors, though; it just says "Crawled
http://rootsitename" and stops there. The content source has "Crawl
everything under the hostname for each start address" checked, and I even
have a crawl rule to include everything under the site.

Any thoughts?

Not sure what you mean by configuring crawl time. Do you mean timeout
factors, or scheduling? I'm just firing manually right now.

What am I missing?

On your "Edit Content Source" page there is a section "Start Addresses". Is it
populated? It should contain the URL's for mysite, sharepoint site and SSP.

It does not have MySite and SSP URLs, but it never has; I removed those.
Before I did the "reset crawled content" thing, I was getting lots of
results in the crawl log; now I just get the main site (at the URL listed in
the Start Addresses section).

Did you try logging into SharePoint as your Search Account to see if they can
access those files?

Actually, yes; that was one of the first things I did this morning :) These
are all list items, and the lists are accessible. No weird broken
permission hierarchies or anything like that.

Also -- I think I mentioned this, but it's been a long day -- before I did
the reset, my crawl picked up everything but search  got nothing. Now the
crawl gets only the root URL.

Was the bizzare error the 2436 Gather error? If so I will pay you $25 for that
dead chicken!

You mean this one?

The start address <http://servername> cannot be crawled.

Context: Application 'SSP1', Catalog 'Portal_Content'

Details:
The specified address was excluded from the index. The crawl rules may
have to be modified to include this address. (0x80040d07)

If so, yes... but I still get it on the other address (not that it matters
since they're both pointing to the same site collection).

Yes, that's the one! The address mine craps out on is the Central Admin site.