Update (Nov. 2012): Several people have noted that the links to the GuruPlug kernel files have broken. I've mirrored them on the blog and changed the links below.
By default, the GuruPlug comes configured to act as an access point, which is probably not what most people want. Unfortunately, getting the plug to act as a reliable wifi client is nontrivial. Most existing tutorials on the topic are pretty in-depth - I'm going to give a quick and dirty method for getting wifi configured and working reliably. It's been working nonstop on my plug for several weeks with no dropouts, kernel panics, or the like.
We're going to install a custom 22.214.171.124 kernel. A prebuilt image and modules are provided on the Plug Computer wiki. A note about u-boot: I'm not sure if the included bootloader on the plugs can boot custom kernels, but if you opt to upgrade it, I recommend the version linked here. I'm not going to discuss the bootloader setup any further in this post.
Login to your plug as root, then download and install the kernel image:
cp uImage-126.96.36.199 /boot/uImage
chown root:root /boot/uImage
And now the kernel modules:
tar xvfj modules-188.8.131.52.tar.bz2
mv 184.108.40.206 /lib/modules/
Now we'll grab an archive of the wifi drivers and firmware, some custom wifi initialization scripts, and the wifi configuration file. We'll store all this stuff in /root.
tar xvzf wifi-files.tar.gz
mv wifi-files/* /root/
There are three main wifi files. The script that runs before Linux brings up the wireless interface is /root/wifi_pre.sh and executes some firmware voodoo to get client mode working properly. The script that runs after the interface is up is /root/wifi_post.sh and turns on a blue LED to give a visual indication that the wireless came up successfully. Finally, there's the wireless settings file /root/wifi.conf.
Open up wifi.conf and input your wireless settings. The file should be self-explanatory and basically fill-in-the-blanks for most setups, but you can read 'man wpa_supplicant.conf' for all the gory details and possible settings.
Now we need to configure Linux to cleanly bring up the wireless interface. Open up /etc/network/interfaces and input the following entry for the wireless interface mlan0:
iface mlan0 inet dhcp
This will cause the plug to DHCP a wireless address -- you can also assign a static IP as you would for any other interface.
Finally, the default GuruPlug filesystem includes a bunch of garbage in /etc/rc.local. Remove any stuff pertaining to wireless (my rc.local has nothing in it at all).
And that's pretty much it. Assuming your plug is properly configured to boot from /boot/uImage, you should be able to restart and Linux will bring up the wireless at the same time it brings up the wired interface (if configured in /etc/network/interfaces). The blue LED will confirm that the wireless came up successfully.
Update (4/11): One problem I've encountered that may bite you later is that the uImage file is (re)generated by updates to the installed kernel package -- which is probably still installed on the plug, even though we overwrote the kernel image. This means that the next time you update your software and an update to the (unneeded) kernel package is pulled down, your custom uImage will be overwritten and the plug will be rendered unbootable the next time it starts. You should be able to avoid this problem by renaming the custom uImage to something else (like /boot/uImage-custom) and then changing the bootloader to boot from that rather than the standard /boot/uImage. Otherwise, when your image gets overwritten, you'll need to manually replace your custom uImage (easy enough if you're running off an SD card).