18ドル.50 shipped on sale at Adorama is a no-brainer. I've installed ArchLinux and they've been running all sorts of things for me - BIND DNS, MySQL backups, Rsync Daemon, LAMP (PHP and MySQL). They are great fun to play with. I make a point to do everything from my phone and iPad to make sure I am "Intel/X86" free. I definitely think ARM is the future. I have a bunch of old USB sticks lying around so these put them to great use.
With an iPad, it takes a total of 20 minutes from start to finish to build a LAMP server with SAMBA/AFP access. Once you enabled SSH, you are pretty much free to do whatever you want with ArchLinux ARM. Go ahead, install WordPress and host a blog from your house for less than 2ドル a month in electricity.
Link: http://archlinuxarm.org/