MacLean's has an article about Google Voice and the potential massive impacts it could have on the cellular industry (which is likely a big part of the reason it wasn't allowed in the iPhone App Story – official story about it duplicating existing iPhone features notwithstanding.)
“Google Voice’s primary offering to consumers—a single, unchanging
phone number—seems innocent enough at first. The service, which began
with one million numbers and is being slowly rolled out by invitation
only in the U.S. (it’s not yet available in Canada), allows users to
take calls from their Google number on as many as six different
existing phones…. To use the service, people still need cellphones and cell plans, but
with one fell swoop Google Voice has already sounded a death knell for
many of the lucrative money-making features that were once the domain
of traditional phone companies…. Even more worrisome for telecom players is the possibility that one
day a tool like Google Voice will also let users make phone calls via
the Internet, using smart phones connected to WiFi hot spots instead of
the carriers’ own cellphone towers.”