I don't think prices will go up as much as people think. Despite what this article says, T-Mobile will still be third in size. They won't pass up AT&T in terms of customers. If the merger does happen, they'd still have 30 million less subscribers than AT&T, so if they want to continue to snatch up market share, then the prices will most likely still be lower. Maybe not forever, but I'd suspect at least the first several years.
AFAIK, to help the merger go through, T-Mobile has promised to not raise rates for the first three years. Given the nature of the beast, I'm sure this doesn't mean that they won't remove features from plans.