Is this thing on?
After a lot of jumping up and down and screaming, I appear to have a crude firewall (using Shorewall on my Debian laptop, which was conveniently at hand) on my new DSL line that I can route through. I can traceroute to google.com and read lj -- that's a good sign. Yes, it's faster. Much faster. Ship it.
Total debug time: about 4 hours. Apparently the Debian version of shorewall.conf is missing the crucial line:
IP_FORWARDING=On
... so of course it wouldn't forward packets. Grump. There were a dozen or so other assorted things to fix in the example config, but that was the big one.
About the only other things I've done today was getting the assorted receipts organized for data entry, and taking a 2.5 mile walk.
Now I'll have to change the DNS entries for my assorted domains, and -- most importantly -- fix my mail configuration so that it will relay through sonic's mail server.