Why "Non"?
Because the French want to be on top, and they don't want the "wrong sort of Europe", that's why "non".

The latest survey shows that 55% of French voters will likely vote no on the 500-page-EU Constitution referendum. The Economist explains,

From a tender age, French voters are taught the virtues of Europe. For political leaders, on left and right alike, Europe has been the means of preserving and projecting French power in a world that was otherwise eroding it. In short, Europe offered comfort: protection from decline; reaffirmation of their social model; the foundation of peace.

This sense of comfort is now falling away. In its place, Europe is increasingly seen as a menace: a destroyer of privileges and a source of new threats. Take the two issues that vex the French most just now, neither related to the constitution, but both overshadowing it: the European Commission's directive to liberalise services, which Mr Chirac ripped apart, just as he had earlier torn up the euro area's stability and growth pact, at this week's EU summit (see article); and Turkey's possible EU membership. The first, introduced by Frits Bolkestein, a Dutch liberal, has become an emblem of French fears about an “ultra-liberal” Europe. There may be genuine concerns about lower wages or safety. But nobody has even tried to explain the merits of the measure, although it was approved by the two French commissioners at the time (one of them, Michel Barnier, is now foreign minister). It has rather become, as one socialist puts it, a symbol of “Europe's drift towards liberalisation”.

The French don't want the idea of the “wrong sort of Europe”. EU Referendum looks at A sunset in Europe:

More likely, appears that Chirac feared that even 10 minutes of Barroso's "liberal" views on French television might cost votes in the referendum, although the French president is known to harbour a "low regard" for the commission chief and his "Anglo-Saxon" views.

However, there are fears that Chrac, by portraying the commission as an ultra-liberal Anglo-Saxon institution, may be fuelling the "no" campaign rather than his pro-constitution effort.

The Telegraph, which EU Referendum quotes, sees these as Hopeful signs from France, but with a propaganda machine in the works,

A "No" vote on May 29 would solve so many problems for Britain that it seems almost too much to hope for. Surely the French can be relied upon to let us down. One thing is for sure: the full weight of the Gallic establishment will be deployed in the attempt to bludgeon voters into submission. The BBC's pro-Brussels sympathies are as nothing compared with those of French television and most newspapers; the state will spend vast sums in the attempt to twist its citizens' arms. Anything that can be done will be done.

It's already started. Last evening's (government-sponsored) France2 newscast inaugurated a series of pro-EU Constitution questions-and-answers aimed to make the viewer vote "oui". Expect a lot more to come.

Japan, however, managed to say "Non" to Jacques,

In fact, Koizumi, leader of a nation renowned for its diplomatic protocols, was uncharacteristically blunt, telling l'escroc that Japan strongly opposed the lifting of a EU embargo on arms sales to China.

On another issue of contention, the siting of the proposed International Thermonuclear Experimental Reactor (ITER) project, Koizumi also refused to yield to the French bully, telling him equally bluntly that Japan would not give up its bid to host the site.

Thank you, Mr. Koizumi.