This is probably a prickly point, given Japan's significant investments into China over the last several decades - but Japan basically has no right to claim doodley squat in Asia, as they've never properly atoned for their invasion of all asia, not to mention their war crimes.
For those of you who claim Japan apologized - the specific translation of the japanese apology goes something like this: We regret your pain.
Japanese culture has many ways to apologize. This is the equivalent of a Chinese person saying "bu hao yisi" after they just slaughtered, tortured, raped and pillaged your family, children, and neighbors. Mao essentially spat at this so-called apology - and with good reason. So perhaps you Japan-o-philes living in China can better understand the deep revulsion Chinese have towards the Japanese government.
I'm a Japan-o-phile - love japanese things, love their culture (a well maintained copy of China's Tang dynasty period with some eccentricities), love their people - but am only too painfully aware of their government's war time atrocities AND their stubborn refusal to take responsibility for their actions. This cannot be sustained and China has clearly stated Japan will atone for this soon (I'm assuming this means before the 100 year anniversary of Japan's invasion of China). Japan's war crimes stained all Asia. The longer Japan waits to sincerely apologize, the less the value of that apology.
Incidentally - all those Korean and other nations' women that Japan turned into slave prostitutes - they cannot receive redress (financial compensation) because of US intervention - essentially giving Japan a carte blanche waiver for all war crimes in Asia. That was mighty arrogant.
Personally, I think China is exhibiting impressively amazing restraint and showing extraordinary civility in negotiating the fate of the diaoyu rocks with japan, but it's definitely a powder keg to be watched carefully. We have an extraordinarily interesting modern version of the three kingdoms playing out before our eyes. China - Japan - USA.
The USA - with all it's postulating, shares a large burden of blame for the general continuing annual displeasure of all east asia (minus Thailand - read up on that for yourself) with Japan's post-modern history in Asia. The USA prevented the war tribunal trials of most of Japan's flagrant war criminals - executing those few that were well known to Americans for crimes against Americans and freeing the rest. Now there's another debatably poor US foreign policy that's STILL having long-term repercussions. Penny rich, pound (£) foolish, as they say.
China's problem is how to take possession of its islands without looking like a bully (aka like Japan in its military heyday). Most difficult if not impossible problem to resolve peacefully with all parties (minus Japan) satisfied, if not utterly happy and content. Japan is lobbying heavily, with US support - by providing aid and other economic benefits to the lesser claimants. Current Japanese activities with Myanmar/Burma should be quite obvious - given the backdrop of Japanese history in the region.
Things aren't as simple as merely the Diaoyu Islands and their alleged natural resource assets and China just played the nationalist patriot card to show Japan and the USA the stakes involved.
Bluff or deadly serious? I think China is deadly serious - as playing the patriot card is irreversible. It's the poker equivalent of "all in" or fully committed.