The "radio" on a smartphone is a complicated mess of interconnected drivers and APIs (what the camera has to do with the phone I'll never know). Regardless, improvements to the radio can improve signal strength, data throughput, WiFi, and somehow unlock 720p video recording, all all the same time. Of course, it could make your phone suck down more battery juice, or even prevent your phone from being a phone at all (by killing your wireless abilities).
The Nexus One was recently released in Korea, and came with a new radio. This radio is apparently agnostic to carrier-specificity (it's reported to work on all varieties of the Nexus One, those in Korea, in the U.S. using AT&T and T-Mobile, and even users in New Zealand ).
Initial reports indicate that most users are seeing marginal increases in 3G speeds, and no noticeable impact on battery life using the Korean radio.
One joker in the forums said the only thing he noticed was that people he was calling had a hard time understanding him with his new Korean accent. ;)
I don't have super-fast 3G coverage in my hometown, but in my tests I also saw an increase in speeds. Ironically the upload speed increased more than the download speeds.
141kbps download speed
81kbps upload speed
203kbps download speed
422kbps upload speed
If you have unlocked your Nexus One and are running a custom recovery (such as RA-Recovery) and you'd like to give the new radio a try you can download the 5.08 radio here; put this .zip on the root of your sdcard and reboot into recovery mode. Flash the radio508signed.zip and reboot. Then reboot again when prompted.
If you want to see what kind of improvements you get with the new radio download FCC Test from the Market, run a test before flashing, then run another test after flashing. Post your results in the comments below.
Source: (XDA Developers)