I absolutely agree that China has been using the North Korean issue in its bid to annex Taiwan, but I seriously doubt things have turned out the way Beijing expected. While China has certainly gotten a lot more sophisticated in its international diplomacy, it still doesn't understand the importance of a good public image, particularly with the US government. By agreeing to play an active role in the nuke issue, the Chinese hoped the US would give them a little bit more flexibility toward Taiwan, perhaps hoping to induce the US government to change its policy to sell weapons to that country, but to no avail. China has been leaning on past and present administrations to get what it wants, namely the continued marginalization of Taiwan and an end to weapons sales.
The Taiwan Relations Act, passed by Congress* in 1979, sanctions arms sales to Taiwan, and despite China's fervent wishes can't be repealed by presidential fiat. This is where China's role in the 6 Party Talks came in. Simply, it was a gambit to A) improve its image in the eyes of the US government to exact concessions on Taiwan, and B) have an active hand in an issue that it can (and has) used to drive a wedge between the US and its allies in NE Asia. While it seems to have succeeded to some degree with the latter objective, the US gov't (particularly the White House) still isn't biting.
But alas, any successes it gained from the 6 Party Talks were wasted when Beijing pushed through the Anti-Secession Law. Conventional Wisdom now says that China prepared the law in the event that Taiwan President Chen Shui-bian's Democratic Progressive Party won it all in last December's legislative elections. When the DPP ended up crapping the bed and remained the minority party in the Legislative Yuan, China's law became something like a rampaging Frankenstein Monster, too big and politically dangerous for even the highest levels of government to stop. No one in Beijing wanted to look "soft" on Taiwan, nor had the political huevos to try to do anything, even after it became apparent in December that no one outside of China was going to buy it. It was almost as if China had locked itself in its room and eaten a Costco tray of bran muffins, washed down with a venti-sized coffee...while no one in the world knew the exact details, everyone knew the end result wasn't going to be pretty. All China could do was hopefully toot the "peaceful unification" horn while keeping the beast under wraps until the rubber stamp People's Congress convened in March.
We're now seeing the results of the CCP's inability to control its image: The US gov't knocking China for trying to upset the status quo in the Taiwan Strait, the international media pouncing on the "non-peaceful" clause in the Anti-Secession Law, and a million marchers in Taipei drawing the world's attention to China's own ineptness. China might be able to buy off the UN but it has yet to be able to convince the US to sell out Taiwan. And now people are beginning to put two and two together, realizing that Beijing didn't give a shit about the outcome of the NK talks, it just wanted to drag them out as long as possible as it tried to convince its critics it was ready to play nice with the neighbors while it continued to put pressure on Taiwan.
*Fortunately for Taiwan, China still doesn't seem to understand the political importance the opinion of a truly functioning and independent legislature like the US's carries. True story from Capitol Hill: a congressional staffer once had to call security to escort some PRC diplomats out of the building after they threatened to cut off all trade with that particular congressman's state if he didn't toe the One China line and withdraw from the Taiwan Caucus. A threat's a threat, and that shit don't fly there. The Chinese have become a laughingstock on the Hill for that stunt.
Also, check out this post and comments over at Richard's site. American liberals and conservatives seem to be together on this point: whatever China's trying now looks awfully stupid.