He's one of the sport's brightest young riders.
Still, many within cycling regard Zabriskie as perhaps the best time trialist in U.S history.
Most recently, he finished second overall in the Tour of California while riding for the Garmin-Barracuda pro team, destroying the time trial course along the way.
But Zabriskie is 33 years old and is expected to compete in the Tour de France not long before the Olympics.
Phinney will not compete in the Tour after competing in the Giro, potentially leaving him fresher for the Olympics than Zabriskie, a seven-time (and defending) national time trial champ and two-time world championships medalist who is one of only four Americans who have led the prestigious Tour de France.
Still, Zabriske's exclusion will fuel speculation that he might be among the witnesses involved in the latest doping allegations against Lance Armstrong.
The U.S. Anti-Doping Agency has filed charges against Armstrong on the strength of eyewitness testimony by "more than 10" riders and team employees from various organizations.
Retired cyclists Floyd Landis and Tyler Hamilton themselves disgraced and banned in doping scandals are presumably among the riders on the witness list, since they have accused Armstrong in the past.
But any others are not known. And USADA said it will not disclose the names of its witnesses, for fear of intimidation.
Zabriskie was among seven cyclists hoping to land an Olympic spot who also had been members of the U.S. Postal Service team with Armstrong and coach Johan Brunyeel. Among them, only top-ranked American Chris Horner was named to the men's Olympic team; veteran George Hincapie was another notable exclusion.
Levi Leipheimer, a Rowland Hall graduate who won bronze in the time trial in Beijing, would have been in that group, too. But he told Velo News recently that he didn't expect to return to the Olympics because he's still recovering from injuries suffered when he was hit by a car in Spain in April.
Indeed, he was not named to the team.
Instead, Timmy Duggan, sprint specialist Tyler Farrar and phenom Tejay Van Garderen rounded out the five-man roster.
"We have a strong team going to London with a solid combination of experience, leadership and young talented athletes who are all capable of standout performances," USA Cycling vice president Jim Miller said in a statement. "Each member of the team is deserving, we're proud to welcome them as a part of Team USA and look forward to a promising Olympic Games."