Can volume automatically decrease during communication activity? A feature of Windows 7 that I really like that it can decrease the volume of system sounds/music by 50%/80%/mute entirely when it senses communication activity, ie. Skype call, Google Voice call, etc.

Is it possible to do this in OS X either out of the box or with a third party app?
Skype has a preference called "Pause iTunes during calls", enabled by default. In "Telephone" it's called "Pause iTunes during call" and iChat pauses iTunes automatically, without any preference. There is no system-wide setting however, but there are not many phone apps remaining than those as well.