From The Apple Wiki

Baseband of an older beta 3G firmware. This is only known and OK to post because it has become semi-public knowledge after somebody on Hackint0sh leaked it. This baseband has been confirmed to be on the iPhone OS 2.0 build 5A331 ramdisk. According to Zibri, it can also be found on the 2.0 beta 7 (build 5A331) iPod touch ramdisk for some reason.

This features prominently on the iPhone Dev Team blog. The post can be found here.