Probably, but since tethering is probably a feature 10% of their data subscribers are interested in, they would also raise the data price on the 90% that don't need it (who are also be more likely to walk away if prices go up.)
Probably, they will do what they are currently doing but in a different way. For example, offering tiers based on data usage: under 100MB plan (aka non-tethering) for $n; over 100MB plan (aka tethering) for $5n.
