Broadband Voice and the PSTN
Several blogs (Om Malik, Aswath Rao, Andy Abramson) write about how the voice quality of SkypeOut is not as good as the voice quality of Skype because Skype uses a wideband codec (the GIPS iSAC codec) for Skype-to-Skype calls but uses G.729a for SkypeOut calls. Aswath comments that it "looks like the interconnect providers still do not see the need to upgrade their gateways to support the wideband codec."
The problem is, even if someone built a VoIP gateway that supported the iSAC codec, the circuit-switched side of the gateway would still be talking G.711 to the PSTN. You could get voice quality approaching that of a PSTN call, but you're not going to get the 8kHz of voice bandwidth that you can get on a Skype-to-Skype call (with sufficient network capacity and performance).
Of course, PSTN-quality is better than G.729a (by about 0.5 MOS points), so by using G.729a Skype is accepting a voice quality lower than PSTN for SkypeOut calls. I would conjecture that they choose to do this because G.711 over IP is a bandwidth hog (about 95 kb/s with 20ms frame size, compared to about 40 kb/s for G.729a). But given that a carrier can achieve PSTN-quality (at the expense of bandwidth) simply by using G.711 for the sessions that interwork with the PSTN, what's the motivation for them to upgrade their gateways to support wideband codecs?
Funny, though - with wideband codecs, sound quality could be a differentiator for VoIP services, rather than a detriment. (I'm by no means the first person to think of this.)