I've been tethering in the past, not often, but no charges. I have been doing so since owning my Hero over a year ago, it was then rooted, but now the Optimus (non-rooted) and it's the same thing. No charges...as msc43283 said earlier, it bypasses the key so you can activate and use the mobile hotspot without paying the charges.
There are other apps that does this as well for the rooted phones, such as "aNetShare" or "Wireless Tehter for Root Users". "Hotspot Widget" is the only one that I know for unrooted that is in the Market that was made for this and is pretty straight-forward. It works the same way.