Even if there is a cap, I would think it'd probably be above 5GB... at least for the fixed, they're trying to go up against the wireline ISPs, so 5GB simply won't cut it.
Don't think they're trying to go against cable. Not with a few billion from the cable companies.
Though if Clear and cable can coordinate their offerings, it'll make things a whole lot less painful for people out of range of cable but within range of WiMAX.