Over at Macworld, Rob Griffiths wrote a great summary of "airport" command. One of the most important things to notice about that article now is the date: September 5, 2008. Like most of the other tips that I found via Google, this one was old enough to be outdated. The 'airport' command changed in Snow Leopard, and no longer supports the -A or --associate argument. Again, note the path you had to travel to even find that command in the first place. Clearly Apple does not feel compelled to offer backwards compatibility for a command found eight levels deep in the System folder.
Strangely, while you cannot associate to (or "join") a Wi-Fi network using the 'airport' command but you can disassociate from (or "leave") all Wi-Fi networks by using "sudo airport --disassociate" which will leave your AirPort card turned on but not connected to any networks.
I went into Google's Advanced Search page and told it to only show me recent results (sidenote: Google does not offer nearly specific enough options for this. Your choices are: anytime, paste 24 hours, past week, past month, past year. What I wanted was "Only since August 28, 2009, the date when Snow Leopard was released.) From there I found a discussion thread on Apple.com which made it clear that -A was no longer possible, but I should checkout "/usr/sbin/networksetup" instead.
networksetup -setairportnetwork Airport 'Your SSID Here' 'Network Pa$$wOrd Here'
did just what I wanted, and it did not require an administrator password or sudo. If you need to get a listing of all of the current Wi-Fi networks which are broadcasting SSIDs, you can use 'airport -s' for a list which will also show what security measures are used.
By the way, if you aren't familiar with networksetup, you should definitely read up on it. It has a lot of features that could come in handy. It also wields a great amount of power, so be careful. The command listed above is fairly innocuous but there is a lot more it can do.