I grab the latest ubunutu of the elinux.org site, and Wifi works out of the box (with a generic Wifi USB device). You need to edit /etc/network/interfaces and make sure that you auto wlan0 and provide WPA passwords and what not.
Do your research on what Wifi devices to use. Many don't work and others are unstable causing agony and frustration like a thousand burning suns. After all you rely on drivers that compile perfectly on ARM.
thx @ChuckMcM I've only read about unstable installs - the only reason I want to try a beagle is the OpenBSD armv7 port http://www.openbsd.org/armv7.html can't see wifi though :(
How did you go installing Linux? What distro? Have you got wifi working?