Getting Pithos up and running on Arch was relatively painless. I was able to build it from source with minimal dependency pain, but I've since ditched that method for using the AUR.

The 'Manual' AUR Method

I'd recomend this method if you are just starting out. It's good to learn the manual AUR ways.

First, make sure to install the base-devel packages in pacman. I use the --needed flag to avoid re-installing existing packages.

# pacman -S --needed base-devel 

Now, pull down the tarball from the AUR into a location of your choosing. I like to keep all my AUR downloads in ~/aur separated into sup-directories called packages for built packages and tarballs for the AUR tarballs.

## If you are keeping things separated like me
## Assuming you've made ~/aur/packages and ~/aur/tarballs
## ALSO: your versions may differ from mine for AUR built 
## packages

$ cd ~/aur/tarballs
$ wget https://aur.archlinux.org/packages/pi/pithos/pithos.tar.gz
$ cd ~/aur/packages
$ tar -xvf ~/aur/tarballs/pithos.tar.gz
$ cd pithos

From here, we can use makepkg to create our tar.xz for installation via pacman.

## Using the -s flag, makepkg will resolve dependencies, 
## given they are available in the configured repositories.
## All the dependencies for Pithos -should- be in your repos.
## There are cases where the dependencies are in the AUR,
## so you have to build them first (more on that in the next
## section)

$ makepkg -s

Assuming that all went smoothly, your last line of output should be similar to

==> Leaving fakeroot environment.
==> Finished making: pithos 0.3.18-1 (Tue Mar 11 08:56:45 CDT 2014)

and you should have a hot'n'fresh tar.xz package.

$ ll
total 268
-rw-r--r-- 1 dauser users 115195 Mar 11 08:56 0.3.18.tar.gz
-rw-r--r-- 1 dauser users 145448 Mar 11 08:56 pithos-0.3.18-1-any.pkg.tar.xz <--- HOT'N'FRESH
-rw------- 1 dauser users    189 May  4  2012 pithos.install
drwxr-xr-x 3 dauser users     60 Mar 11 08:56 pkg
-rw------- 1 dauser users   1074 Dec  5 17:17 PKGBUILD
drwxr-xr-x 3 dauser users     80 Mar 11 08:56 src

From here, it's a simple sudo pacman -U pithos-0.3.18-1-any.pkg.tar.xz

The Slightly Less 'Manual' AUR Method

For most of my AUR needs, I've started using pacaur which is available in the AUR. To build and install it, you will also need cower. This is a case where your dependency is not in your official repos, so you have to build cower before building pacaur.

## If you are keeping things separated like me
$ cd ~/aur/tarballs
$ wget https://aur.archlinux.org/packages/co/cower/cower.tar.gz
$ cd ~/aur/packages
$ tar -xvf ~/aur/tarballs/cower.tar.gz
$ cd cower
$ makepkg -s
## After that completes successfully
$ sudo pacman -U cower-11-2-x86_64.pkg.tar.xz

Now you can build pacaur.

## If you are keeping things separated like me
$ cd ~/aur/tarballs
$ wget https://aur.archlinux.org/packages/pa/pacaur/pacaur.tar.gz
$ cd ~/aur/packages
$ tar -xvf ~/aur/tarballs/pacaur.tar.gz
$ cd pacaur
$ makepkg -s
## After that completes successfully
$ sudo pacman -U pacaur-4.1.25-1-any.pkg.tar.xz

You can now use pacaur to install pithos. The syntax is pretty much the same as pacman.

## Searching
$ pacaur -Ss pithos
aur/pithos 0.3.18-1 (26)
    Pandora Internet Radio player for GNOME
aur/pithos-git 1:379-1 (39)
    Pandora Internet Radio player for GNOME
    
## Installing
$ pacaur -S pithos

That's it! Pithos should now be installed. It took two AUR builds instead of just building the pithos package, but using pacaur can save you some time down the road.

You can do a lot more with pacaur, but be sure to read that man page!

Sidenote

If you are a Pandora One subscriber...and you should be...getting the Audio Quality set to high might involve some extra gstreamer packages. I wish I could tell you which one is the needed package...but I've not taken the time to track it down, mainly due to laziness, but what follows is the list of gstreamer related packages I have installed on my Arch build.

$ pacman -Qsq gstreamer
gst-libav
gst-plugins-bad
gst-plugins-base-libs
gst-plugins-good
gst-plugins-ugly
gstreamer
gstreamer0.10
gstreamer0.10-bad
gstreamer0.10-bad-plugins
gstreamer0.10-base
gstreamer0.10-base-plugins
gstreamer0.10-ffmpeg
gstreamer0.10-good
gstreamer0.10-good-plugins
gstreamer0.10-python
gstreamer0.10-ugly
gstreamer0.10-ugly-plugins