All right... I give up. I've been trying to make this happen for the course of an hour now and it's just not cooperating with me, so I'm obviously doing something wrong

Here's what I'm trying to accomplish:
Execute a script as a non-root user at boot time. The script lives in /home/sbrown/scripts and has the executable bits set. If I run the script locally in the shell it behaves as normal, but will not at boot time It's a very simple 2 line script:
#!/bin/bash
# Starts IRSSI in a screen session
sudo -u sbrown screen -S irssi irssi

Here's WHAT DOESN'T work:
I put this in /etc/rc.local: sudo -u sbrown /home/sbrown/scripts/irssi_exec
I also tried just copying the script to /etc/init.d/ and chowning it to root then ran update-rc.d irssi_exec defaults and rebooted. Still not working.

This is on a Debian Squeeze box... what am I missing? Surely it should be simpler than this

Assuming that you use Gnome, you should have something like "Session and Startup" in your preferences menu. There you can add autostart-applications. Remember, if you use screen, you have to start the applications in a terminal.

Nope, no X on this server, purely CLI. Is screen the potential hangup here?

If you don't use X it should be even simpler. Just add your command to your ~./bashrc.
Or you change your /etc/inittab (be careful with that, make a backup copy first and have a live-medium at hand to fix it if something goes wrong) to start it on a virtual console.