Looks like it is indeed possible to connect a Squeezebox wirelessly without a wireless router!!
http://wiki.slimdevices.com/index.php/Beginners_guide_to_wireless_network_problems
Wireless Ad-Hoc networking
Also known as Independent Basic Service Set (IBSS) wireless networking. The Squeezebox can connect to another wireless device without a wireless router or wireless access point.
For Macintosh systems, on the Airport menu you will find a section about halfway down entitled "Computer-to-Computer Networks". Click on the "Create Network..." option, and it will walk you through the process of creating a network. Write down the SSID (network ID) you enter, so that you can be sure your other devices are connected to the correct network.
For Windows XP systems, Microsoft has a web page describing how to set up an ad-hoc network at
http://www.microsoft.com/windowsxp/using/networking/expert/bowman_02april08.mspx