I have the GRL90 release, with baseband D720SPRKE5 and wifi tethering works. The only difference from my phone and stock is i am using Netarchy's 1.3.7 CFS kernel. I just tossed a droid incredible into airplane mode to kill all radios, turned on WiFi, connected to my NS4G, and browsed multiple websites.
Looks like a custom kernel will fix the tethering issue. Besides Netarchy's kernel, i read people have the same results flashing the matr1x kernel. Also have read that 4G tethering works with these kernels now after this update but 4G doesn't penetrate my office so i cant test.