GrandCentral is a widely praised (if buggy, according to some) service that enables one phone number for life, and allows you to select which phones ring. Listen in before you pick up? Not a problem.
GC also has a mobile service, with a suite of services best described via this illustration:
My take on Google:Grand Central: maybe this acquisition will enable the sometimes-overlooked GoogleTalk service to finally enter the PSTN (Public Switched Telephone Network) call-termination arena with as much gusto as Google is capable of when they put their minds to it.