Based on his comments I thing those of us on 3G are safe. The Tiered plans will be for 4G which will only be used via USB adapter for laptop/desktops for now. 4G phones aren't expected until sometime next year.
I think they're content to keep things for 3G as they are for now if you read into his initial comments on Tiered pricing.
Is it coming for 3g? I think perhaps it may, but I think they're more focused on 4G as the data rates for 4G are significantly higher versus 3G.
That's why I'm not worried at all. As it stands I'm only using about 250Meg a month on Average, so even whenever I do upgrade to the X I might see my usage triple or quadruple initially as I download apps then settle down as I get set into what I like. So depending on what they may set the Tier up I could save money, could stay the same, could spend a little more. It will depend.