I got an Arduino Yún from Adafruit a couple weeks ago but have been slow in getting started with it. In trying to get the TermperatureWebPanel example to work, I ran into a couple issues. These things are noted in the documentation at one place or another but I didn't get things right at first.
The TermperatureWebPanel example requires an SD card. Be sure to create an arduino directory in the root of the SD card. Under the arduino directory, create a www directory. Reboot the Yún after inserting the SD card. The SD card should be linked automatically to /mnt/sd.
If you are working on a desktop computer that cannot connect to the Yún wirelessly, you can use the ethernet interface to upload the sketches.
I have been able to get the example to work when viewed in Chrome and Safari, but it does not work for me when accessed using Firefox or IE.