I just answered my own question, I was monitoring my Sprint account online under Usage Details and was recording my unlimited Sprint Data. I definitely did not see any additional charges and all of my data usage thus far (I'm currently tethering) seems to be included.
This definintely makes this phone a keeper for me as I had a 30 day trial to decide whether to keep this phone or not. Being able to tether is a big plus, I hope it doesn't go away from what I've been reading about Sprint decommissioning tethering for all their phones without a tethering plan in 2010.
What's even great about this phone is being able to still receive text messages and emails on the phone while tethering. I couldn't do this on my Blackberry before, very nice...