I'm going to guess by "Radio" you mean the MXC300. As far as I can tell from the Universal, Data is sent over USB, while the device is actually CONTROLLED by serial over an SPI link. (An internal Null Modem link?) Looks to me that the RPU emulates a serial modem over the SPI, while larger amounts of data, such as broadband internet data and images are fed over the USB, which is connected by a CPU-host to RPU-client link.
That, anyway, is how HTC do it. Looks good to me, as the MXC300 has USB2-Client, and if we throw wifi onto CF (Memory bus! >.< XP - Neko, THAT time was a joke.) that gives us a spare USB2-480. If we flash the CPU Bootloader over USB and then lock it RO, there's nothing to stop us doing the same for the RPU ROM, although since it's not going to be as much of a security risk - It'll be flashed over the USB RPU-CPU link and only flashable from inside the PPZ OS- I don't think we should make it a permanent rom like the PPZ bootloader. After all, if we can't get a 3G stack immediately, then it'll have to be reflashed later (With a simpler ROM in the meantime to provide GSM, GPRS and EDGE.)
CF-ATA hooray! That gives us our good fast-MDD support.
EDIT: Dual processor. . That wouldn;t work if you're talking MXC and iMX. The processors then run a shared OS and tasks, not a Radio ROM and a PDA Rom. And if you mean 2xiMX, then I'm starting to seriously question that. to have a phone, we're talking both a CPU and RPU as it is, and 3 processors looks to be a BAD battery drain.
Also, while I'm editing, if anyone wondered, RPU = Radio Processing Unit, or the MXC300-30 Phone chip.