Rooted Incredible WiFi Tethering Not Working
Did you download Wireless Tether from the Market or from http://code.google.com/p/android-wifi-tether/ ?

If you are running Android 2.1 you need to take the extra steps listed here:
http://code.google.com/p/android-wifi-tether/wiki/Setup_HTC_Desire_Incredible

If you are running 2.2 then in theory it should just work.  

I would first try the latest release candidate here:
http://code.google.com/p/android-wifi-tether/downloads/detail?name=wireless_tether_2_0_5-RC2.apk&amp;can=2&amp;q=

Let me know how it works out.