With the number of functionalities that require dedicated hardware in mobile phones rising and rising, it's important to integrate as many of them as possible on a single chip to save space and power. In the past mobile devices used a classic radio chain: A receiver/power amplifier, an analog modem chip and a baseband (CPU and DSL) chip. These days it has become possible to integrate the analog modem chip and the baseband chip on a single die and products such as the Broadcom BCM21331GSM/EDGE chip are now used by major phone manufacturers such as Samsung and Nokia for their 2G GSM/EDGE phones. I've looked a bit around and I haven't seen a similar product for combined 2G/3G phones yet. But that might just be a matter of time now. Maybe the current Qualcomm Snapdragon platform goes into that direction!? If I understand correctly, though, this platform still needs an external transciever chip, something that Broadcom has built into their single chip solution already.
If you have further info on that, please leave a comment, I'd be interested to lean more.