We just received a few more internal training documents that further explains the new Unlimited plans that Sprint will be launching this coming Friday. Of course, that Sprint Unlimited Guarantee is only available for the "unlimited" part of any plan you sign up for, and it doesn't mean the rate won't change over time. The guarantee is also only applicable to the new "Unlimited, My Way" plans, and not to the existing "Everything" plans, even if those "Everything" plans have unlimited data. It doesn't apply to tablets, just phones, and is non-transferrable to another subscriber.
Existing Sprint customers who want to swap out their plans to the new ones may do so without extending their contract, though new lines do require a two-year service agreement. To those who have employer discounts, the carrier will discount the monthly data charge for each line rather than the primary rate plan. However, it also states that the My All-In plan (the one that bundles unlimited talk, text, data and a 5GB mobile hotspot) is not eligible for any discount, employer or otherwise.