People can visit Google's and buy a full-priced or subsidized version of the phone for T-Mobile's network, or a full-priced version for AT&T's network. The site refers interested Verizon customers to the Droid Incredible, another Android phone from HTC.

It makes of Sprint, which announced in March that it would support the Nexus One. The operator now says the phone won't be coming to Sprint given the upcoming availability of the , an Android phone that will run on WiMax as well as 3G.

When Google unveiled its phone sales strategy in January, it envisioned allowing people to choose a phone, pick an operator and service plan, and check out. But even then experts worried that given the variety of technologies and frequencies used by operators in the U.S., that vision would be hard to realize.

"I wouldn't give up on them, but it's a hard thing to achieve in the U.S. cell phone market," said Allen Nogee, an analyst at In-Stat. "In Europe, where they use the same frequencies and technologies and users can just trade SIM cards, they can do that, but in the U.S. it's not so easy."

Even in Europe, however, Google is facing some challenges. For instance, Vodafone agreed to carry the Nexus One but is selling it through its own stores, rather than through Google's Web site.