I've not seen any phone with a baseband on the same chip (or at least, that shared any of the resources of the primary cpu. FCC rules makes that, as the GP mentioned, extremely tricky if possible at all.
What phones have you seen that do have a integrated baseband?
As for FCC rules, in general it's safe to assume that the regulated are smarter than the regulators. The baseband core is probably "logically separate" or something to meet FCC regulations.
