I have been able to tether without rooting the phone by disabling the NAI. Since the tethering is done with the native hotspot app it is in infrastructure mode.
Download and install Launcher Pro from the market. Its free.
Use Launcher Pro or any other Launcher as your default.
Long press an empty space in your home screen.
click shortcut
click activities
scroll down to com.lge.hiddenmenu
expand com.lge.hiddenmenu
click Data Setting in com.lge.hiddenmenu category
click DUN NAI
and under extend modem change it to no
This will allow you to tether without the charge from Sprint.
I found that this setting has to be changed ever time the phone restarts.
Not a big deal since you create a shortcut with launcher pro
It finally works. My first try on my newly arrived marquee, it was not quiet working.... I was able to create the hot spot, but my iPad just waited forever to get the IP address.... then I gave up.... NOT finished my story yet...
Then I decided I want to have a fresh start, maybe resell this phone....so I reset my phone back to factory setting ( setting--> privacy-->Factory data reset).
Next day, i just want to give another try before I put it on ebay. Then magic happened.
It worked. My guess is, the Sprint put a lot of road blocks when I activate my phone. When I reset my phone, I removed those blocks. Now I love my phone. I can use to do everything PLUS it can be a hot spot.