I can't speak for Sprint, but on AT&T (my provider) tethering is NOT included in the unlimited data... you must purchase a tethering plan on top of the unlimited data plan... so it would seem that for some providers, at least, unlimited data isn't really unlimited data...
It's definitely extra. From the Sprint rate plans:
Simply Everything
GPS: Unlimited maps, turn-by-turn directions and more with Sprint Navigation for Sprint and PowerSource phones, or TeleNavxxx8482; for Nextel phones. GPS reliability varies by environment.
Sprint Mobile Email: Access multiple personal email accounts in a single application on featured phones. Providers include: AOL, Windows Live (including MSN/Hotmail), Yahoo! and Gmail.
Sprint Mobile Email Work: Access to corporate email on feature phones (non PDA devices) through Outlook Web Access (OWA) or a desktop redirector. If your company uses Lotus Notes Domino, you can obtain access to that email on your phone through redirector software that can be downloaded and then direct mailed to your phone.
Receive corporate email on Microsoft ExchangeSM 2000, 2003 and 2007 and on Lotus NotesSM Domino versions 6.0, 6.5 and 7.0 through a desktop redirector software.
BlackBerry® Internet Service (BIS): Manage up to 10 different accounts with a simple three-click email setup, and IM on the go with Yahoo!® Messenger, AIM®, Google Talk and BlackBerry Messenger. Please note: BlackBerry Enterprise Server (BES), which provides secure corporate email access and access to corporate IT administration, is not included and available for an additional cost.
Music Premier: Take your music with you and use your phone as an MP3 player and radio. Enjoy streaming radio channels, featured local radio stations from across the country and 99¢ wireless music downloads. For the full radio line-up, visit sprint.com/radio.
TV Premier: Watch shows, sports, news, movies and more, choose from: exclusive content, short clips or full-length episodes. For the full line-up, visit sprint.com/tvguide.
Messaging: Unlimited nationwide, text, picture and video messages to anyone on any network.
Direct Connect® and Group Connect®: Unlimited instant connections, nationwide and at the push of a button. Group Connect is limited to 20 participants on the same network.
Mobile to Mobile: Only available for calls directly between Sprint phones and/or most Nextel phones (not through Voicemail, 411 or other indirect methods). Not available where a Sprint phone is roaming/off the Sprint National Network. Phone as modem or tethering: not included.
Corporate, government or special discounts: Do not apply.