Spotify is a digital music service that gives you access to millions of songs.
This Internet music service allows you to select any song in it's database and play it for free. This service was recently only open in areas of Europe, and has recently (07/17/2011) been opened up to the United States area. The only client currently available on linux is supported on Debian and Fedora distributions, while they do recommend use under wine.
If you are interested in using Spotify under Archlinux, you must ensure that you have Wine installed on your system. In addition to installing wine you will need to take a moment to configure it for the user who will be running the application.
Depending on your choice of architecture it may be necessary for you to enable the multilib repositories, to do so you will need to edit your pacman configuration file (as root).
# nano /etc/pacman.conf
Once you have the file opened you will be editing line 82 and 83, removing the pound sign from the start of both lines. After you enable the multilib repository you will need to install wine.
# pacman -S wine
When wine is installed you will need to change some configuration settings using the winecfg application on your every day user account (not root).
After launching the winecfg application you will be presented with multiple tabs that can assist you in tweaking the performance of the emulator. However for this purpose your main focus will be the Audio tab.
While under the audio tab, you will enable either ALSA or OSS driver by clicking the check box next to them, depending on what software you prefer to use. Also note that the hardware acceleration will need to be changed from Full to Emulation. When done you may exit the winecfg application.
Failure to perform the above task will result in the inability to hear playback.
Obtaining Spotify can be done by registering for an account on their Website, the application does not offer in-app registration.
However you can obtain the application prior to registering by using the following URL. 
After you have registered and downloaded your copy of the installer you will need to run the application through wine, depending on your setup you may be able to run the application by right clicking the file. If not terminal will work just fine, as long as you run the below command in the directory of your download.
# wine Spotify\ Installer.exe
Once the application is successfully installed you may run Spotify by using one of the following commands in terminal, or in the ALT+F2 launcher:
If you use a x86_64 copy of ArchLinux, you'll have to run it like this:
# wine "/home/username/.wine/drive_c/Program Files (x86)/Spotify/spotify.exe"
If you use a x86 copy of ArchLinux, you can use this command just fine:
# wine ~/.wine/drive_c/Program\ Files/Spotify/spotify.exe
If you have any additional problems, I recommend setting the winecfg to Windows XP or Windows 7 emulation.
Get Spotify from the AUR
There is also a copy of Spotify available in the Archlinux AUR, and you can download this application by using Yaourt
# yaourt -S spotify