Just had two fun days of troubleshooting this by working together with Adobe/Macromedia support, and found the solution.
Here’s the original issue: could not set up a new data source connecting locally (localhost, 127.0.0.1) to SQL Server 2000 Standard running on the web server; kept getting a SQL Exception error. Was told SQL just wasn’t listening on port 1433, or any TCP port for that matter, even though TCP/IP and Named Pipes were clearly enabled in the SQL Network Config Utility. Even in the registry, port 1433 was specified, yet I could not connect to SQL on TCP by any means. I couldn’t even telnet to the machine on that port.
Turns out that even though I’d upgraded SQL Server 2000 to SP4, I needed to downgrade to SP3. Still doesn’t make sense, after all, MS SPs are supposed to be roll-ups, but hey, that’s what worked. Luckily, the server I was working was running on VMware, so I reverted to a snapshot I took after I installed SQL and before I upgraded to SP4. Installed SP3, and was able to set up the data source immediately! Something to keep in mind if you’re in the same boat.