This paper which was released in July 2008 is freely available on their web site and is an excellent overview of the state of the codec world and the potential for better-than-PSTN-quality IP telephony. It takes a well balanced view of existing codecs, their ability to convey audio bandwidth, latencies, target applications, network bandwidth and processor load requirements. It gives a significant amount of detail while presenting the information in language that even a non-technical manager will appreciate.
If you have any interest in the future of IP telephony it’s definitely worth a read.