Ok, just went back and checked. I used my phone as a tethered modem for 12 hours across 2 days 3 weeks ago. My bill shows the kb, and also shows no charge, as it did last time I needed to do this.
Per Sprint, as of November 2008, if you have a Blackberry AND the everything data plan, tethering is included.
What they've changed since then, I don't know, but it would be worth it for the poster to check.