Iraq War veteran and former Army Lt. Dan Choi was discharged from the National Guard in July after outing himself as a gay man, in violation of "don't ask, don't tell."
But with the controversial military policy no longer in effect after a federal judge issued an injunction, Choi headed to a Times Square military recruitment center in New York Tuesday to re-enlist.
"Today is a great day we can all celebrate," he said after filling out the paperwork. "I'm very excited to be in service to this country."
Gay advocacy groups trumpeted their plans to test the new rules at recruitment stations around the country, as the Pentagon told military recruiters they have to accept applicants who may openly volunteer that they are gay.
Pentagon spokesperson Cynthia Smith said, however, that recruiters have been told to inform openly-gay applicants that a reversal in the court's decision on the injunction against "don't ask, don't tell" may occur.
Recruiters have also been told not to bring up an applicant's sexual orientation themselves.
In the week since Federal Judge Virginia Phillips' order, imposing an immediate, worldwide ban on enforcement of the policy, the Pentagon has tried to determine what impact the shift would have on its operations.
It took officials two days to put together guidance to legal officers and the service chiefs, saying the Defense Department would comply with the judge's injunction while the Department of Justice weighs whether to appeal the decision.
Gibbs: Obama Will Push for Repeal in Lame-Duck Session
On Thursday, the Justice Department filed a request with Judge Phillips, asking her to stay the injunction pending appeal.
Clifford Stanley, the Undersecretary of Defense for Personnel and Readiness, warned Phillips in the filing that her injunction forces the military to "suddenly and immediately restructure a major personnel policy" during wartime with potentially serious consequences. "The stakes here are so high, and the potential harm so great, that caution is in order," he said.
Still, Phillips is not expected to grant the government's request for a stay of the injunction. She first ruled on Sept. 10 that the military ban on openly gay and lesbian service members is unconstitutional.
Meanwhile, many advocates for gay and lesbian service members are warning them not to come out of the closet just yet.
"During this interim period of uncertainty, service members must not come out and recruits should use caution if choosing to sign up," said Aubrey Sarvis, executive director of the Service Members Legal Defense Network. "A higher court is likely to issue a hold on the injunction by Judge Phillips very soon. The bottom line: if you come out now, it can be used against you in the future by the Pentagon."
White House press secretary Robert Gibbs said President Obama believes that regardless of the legal outcome of the case, the policy will end under his watch.
"The president will push for defense authorization to be passed, containing that provision, when the Senate comes back for the lame-duck," he said.
"I think, if we can get past the procedural hurdles, that a majority of the U.S. Senate believes as the president does: That it is time for this policy to change."
The House approved a repeal as part of the annual defense budget earlier this year, but the Senate failed to approve the measure in a vote last month.
The Obama administration, which opposes "don't ask, don't tell," has preferred a legislative repeal of the law to a battle in the courts.
ABC News' Devin Dwyer contributed to this report.