[guessWhy]

Why?

Verisign merely signs your certificate. It does not even know your private key and hence also can't pass it to governments.

[finnw]

It is not necessary for the government to have your key; They can impersonate your site (using their own key) if they have the cooperation of the CA.

[jlgreco]

The trick is that if this were being done on a regular/wide basis, people would notice and have pretty incontrovertible evidence of it.

I have no doubt they can do it, but it seems to be consigned to their bag of tricks for special occasions.