From the XDA forums:
I just learned tonight that as of Thursday (03/24), 3 days from now, 4G data plans will be capped.
The new cap will be 5GB. Pricing overage is not specified just yet, I would assume the standard $10/GB overage.
This is for any NEW customers and any upgrades done AFTER this day. Anyone who already has a TBolt is grandfathered in (myself included). Spread the word, and let your friends/family know should they need a truly unlimited plan.
I would expect an announcement tomorrow sometime. Just wanted to let you guys know.
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Take it with a grain of salt, but it wouldn't surprise me.