At least compared to the previous iPhones. But fear not -- nothing in unhackable.
iPhone 4 Appears to be Tougher to Unlock : Read more
iPhone 4 Appears to be Tougher to Unlock : Read more
...then it gets rather technical. The baseband is segregated from the main applications processor and communicates with it over i2s, spi, and uart lines. The main processor controls it with (primarily) AT commands over some other transport protocol over the SPI and UART. When you're in a call, the BBP communicates directly with the audio codec over i2s so even if the main OS is freezing up, the call will still go on uninterrupted.
The other advantage is security since if someone cracks iOS, the BBP is still separate. The two don't share any memory, so that's a security advantage.