I just came across a free WiFi tethering application called FoxFi, which is available from the Android Market. This enabled WiFi tethering for non-rooted phones. Version 1.31 is still a beta, and I've only downloaded it a few minutes ago. But I am connected via my phone right now and it seems to work.
https://market.android.com/details?id=com.foxfi
https://market.android.com/details?id=com.foxfi