First, as for the import, Scotus Blog notes:

The ruling in Al-Bihani v. Obama (Circuit docket 09-5051) was the first by the Circuit Court to directly apply the Supreme Court’s 2008 decision in Boumediene v. Bush creating a constitutional right for Guantanamo Bay detainees to challenge their captivity. Unless reviewed and overturned either by the en banc Circuit Court or the Supreme Court, the new decision will control how scores of detainee cases are resolved in District Court in Washington.

....Although the opinion for the majority represented the views of only two members of the Circuit Court, it is a decision that is binding on any other Circuit panel that hears a Guantanano detainee case raising issues of detention power and courtroom rights. It could be overturned at the Circuit Court level only by a majority of the en banc Court. Al-Bihani’s lawyers have the option of seeking such review, or taking the case on to the Supreme Court.

I particularly didn't like the following: Footnote 2 on page 10:

...we need not rely on the evidence suggesting that Al-Bihani attended Al Qaeda training camps in Afghanistan and visited Al Qaeda guesthouses. We do note, however, that evidence supporting the military’s reasonable belief of either of those two facts with respect to a non-citizen seized abroad during the ongoing war on terror would seem to overwhelmingly, if not definitively, justify the government’s detention of such a non-citizen. Cf. NAT’L COMM’N ON TERRORIST ATTACKS UPON THE UNITED STATES, THE 9/11 COMMISSION REPORT 66–67.

As to the standard of proof required to detain a foreign citizen indefinitely: The court not only endorses a "preponderance of the evidence" standard, it suggests an even lower standard like reasonable suspicion might suffice:

The question of what standard of proof is due in a habeas proceeding like Al-Bihani’s has not been answered by the Supreme Court. See Boumediene, 128 S. Ct. at 2271 (“The extent of the showing required of the Government in these cases is a matter to be determined.”). Attempting to fill this void, Al-Bihani argues the prospect of indefinite detention in this unconventional war augurs for a reasonable doubt standard or, in the alternative, at least a clear and convincing standard.

In refusing to find the "preponderance of evidence" standard unconstitutional, it adds:

We emphasize our opinion does not endeavor to identify what standard would represent the minimum required by the Constitution.

And then another footnote #4:

In particular, we need not address whether a some evidence, reasonable suspicion, or probable cause standard of proof could constitutionally suffice for preventative detention of non-citizens seized abroad who are suspected of being terrorist threats to the United States.

(The opinion notes that such a standard, used by a British court, was overturned as inconsistent with European Union law.)

In holding that Guantanamo detainees have more restrictive habeas rights than U.S. petitioners, it says:

Habeas review for Guantanamo detainees need not match the procedures developed by Congress and the courts specifically for habeas challenges to criminal convictions. Boumediene’s holding explicitly stated that habeas procedures for detainees “need not resemble a criminal trial,” 128 S. Ct. at 2269.

...Detention of aliens outside the sovereign territory of the United States during wartime is a different and peculiar circumstance, and the appropriate habeas procedures cannot be conceived of as mere extensions of an existing doctrine. Rather, those procedures are a whole new branch of the tree.

...Unlike either Hamdi or Al-Marri, Al-Bihani is a non-citizen who was seized in a foreign country. Requiring highly protective procedures at the tail end of the detention process for detainees like Al-Bihani would have systemic effects on the military’s entire approach to war.

And another footnote:

Both Hamdi and Al-Marri involved American citizens or legal residents; the procedures to which Americans are entitled are likely greater than the procedures to which non-citizens seized abroad during the war on terror are entitled.

The Court says Al Bihani was a cook for the 55th Arab Brigade, which fought alongside the Taliban while the Taliban was harboring al Qaeda. He claims he was just a civilian contractor providing services.

Even assuming, as he argues, that he was a civilian “contractor” rendering services... those services render Al-Bihani detainable under the “purposefully and materially supported” language of both versions of the MCA. That language constitutes a standard whose outer bounds are not readily identifiable. But wherever the outer bounds may lie, they clearly include traditional food operations essential to a fighting force and the carrying of arms.

So, if you're not an American, be careful whom you cook for overseas. What if you cater an affair that is being sponsored by a group that is associated with or supports the goals of one of our enemies and there are firearms at the event? Is that enough to authorize your transport halfway across the world to be held for years in indefinite detention at Gitmo? Using a lesser standard of proof than an American would receive, possibly even "reasonable belief", it just might be.