The 777 is a fairly common long distance aircraft. Would it be possible to give it a new paint job, and then in similar fashion to the OP, fly in the shadow of an existing flight?
For example, you paint it up like a Lufthansa 777, and then try to follow one of those flight paths at some point between London and New York. If anyone gets a visual on the mystery plane flying into US airspace, it looks like a commercial flight simply not responding. Not quite as suspicious as a jet or military aircraft. Would they shoot down a commercial 777 that's flying a standard route, with no indication of it being hijacked, or a threat?
I'll admit, the above idea is absurd, but this entire situation is making little sense. I hope we get some answers one day.
Edit: A lot of people are saying in other comments that hijacking a plane isn't an effective method of acquiring a 777, or stealing cargo, or kidnapping people. However, it's been 10 days, and we simply know the plane disappeared. That seems pretty effective, a 777, 230 passengers, and cargo, all gone, and no one even knows if they were targeted, or if the plane malfunctioned and crashed in the ocean. If any of the above were of value to someone, it wasn't only effective, it was near perfect.
(Luckily they used the "wrong" anthrax, so not much harm done).
The same organisation also did odd things in Australia. They made sarin and VX gas, tested that on sheep, and the. Used the gases in several assassinations.
They released sarin in Matsumoto, killing 8 people and harming 200. They didn't claim that attack.
Aum Shinrikyo are well funded, have smart people working for them, and are happy to murder people.
Also, IIRC Aum never used anthrax (although they did stockpile it), only sarin and VX, which are chemical agents, not biological.
Here are some links to the anthrax stuff:
http://www.cnas.org/files/documents/publications/CNAS_AumShi...
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC88589/#!po=5.952...
This attack could have been significantly worse.
China isn't exactly a country I would mess with, I think they would be about as restrained as Russia. Terrorist would expect a big response from them, or do they see them the same as pre-Bush US?
Even if they got the plane for later use, just trying to use it would be amazingly difficult. However we did tell them how we knew it flew so they do know what to knock out to fly it again.
As for claiming responsibility, disappearing a plane and not claiming it is simply amazing, do it twice and aviation will shut down in that part of the world, do it three times, well hell.