Google Voice, with its deep integration into Android and newly released iPhone native app, along with all the infrastructure that Google has built around the service, gives the company a leg up if it wants to move in that direction.
They are also other signs that Google might be interested in becoming a carrier, such as the fact that in 2008, Google put in a bid to buy wireless spectrum to provide mobile Internet access (which went eventually to Verizon), and its moves into experimental broadband with its Google Fiber project.
However, the fact that Google has already admitted that it makes the bulk of its revenue from Android on search, it would probably take Verizon, for one, mere seconds to substitute Bing onto its Android handsets. It's already put Bing into a few of them, much to the dismay of end users.
Of course, as SAI points out, if Google were to attempt such a move, it probably wouldn't get federal regulatory approval. Both the CTIA and Verizon spend far more money on lobbying than Google does.
There would be a host of other obstacles in Google's way, including its unfamiliarity with all that's required to be a wireless carrier. Instead, as Forrester Research analyst Jeffrey Hammond said,
"While I think Google could become a mobile provider, I'd view it as a nuclear option."In other words, as long as the current set of carriers don't get in the way of Google's ambitions to be the hub for all things Internet, all will be well. If things change, you could be looking toward a Google-branded Android phone, with that branding referring to Google as a carrier, not just for a pure Android phone like the Nexus S.