Im pretty sure they did block it from their build of froyo. Its probably cuz they want you to pay extra since they have a tethering plan which adds an extra $20-30 usually for 2gbs of data tethered. I have an unlocked dell streak and was able to get the froyo that dell released and i have tethering on my phone which is with AT&T. Problem is im so worried to use it cuz i dont want them to monitor and catch me using is at a mobile hotspot and then tack on some ridiculous charge on my bill, although I have used it here and there to test it out.
Alpine White Mini 9, 2Gb ram, 64Gb Runcore SSD, 1.3MP cam, internal BT module, Dualboot Win7/OSX 10.5.8
Unlocked Streak, Streakdroid v.1.9.0
XPS M1330, 4Gb ram, 320Gb HDD, 128Mb Geforce 8400M GS, Wireless N, internal BT module, Slim and Brite LCD, Windows 7 Ultimate