Apple are one of the exceptions, which I should've stated in my post, sorry; I was replying to the previous poster's claim that *all* phones work this way, which is most certainly not true (and a vanishingly small proportion if you count low-end cellphones as well).
Apple, HTC and many of the Blackberry devices are dual chip, but these are a very small proportion of the global smartphone market which is dominated by Nokia and by the large Japanese manufacturers (who all use single chip designs for most or all of their devices).
The analogue components of the baseband (the actual radio) are a separate issue from the processor that runs the baseband communication stack; single chip phones still have a separate radio chipset, but it only handles the signal processing domain, not the protocols used to communicate with the cell tower. Radios are off the shelf components: some include a baseband processor, some do not, but both varieties are available from chip vendors and ones without a processor are significantly cheaper.
I've looked inside plenty of cellphones, by the way; I'm a realtime OS kernel developer for a major cellphone manufacturer, and have to deal with issues from the people developing the baseband stack quite often :)
For an example, pick any Nokia phone released since about, hm, 2002? Only Nokia's very earliest smartphones used a separate baseband processor. Motorola, Fujitsu, Sharp, Samsung, and probably many others also use single chip systems for the majority of their modern products.