First, some questions. What does your script need to do? Do you really need it to run at boot time or would it be better for it to run when you log in? (Of course, if you're the only user and you're using automatic login, boot time and login time are the same.)
If it can run at login, see the Login Items pane of System Preferences.
If you really need to run at startup, see the System Overview document in the developer tools documentation. You'll need to write a shell script, although it can still run an AppleScript via /usr/bin/osascript.