Is an hypothetical "chance" attack possible?

[Xunie]

People discussed that there is an element of "chance" or "luck" involved in generating a block.
So, one day I might generate two blocks while another day I might generate none.

Now the network is build around people generating blocks which seals in the transactions etc.
And since a double spending attack is possible by 'reversing' the block -- which requires significant CPU time --,
is the 'double spending attack' based a bit on chance, or not?

If so, then an attacker wouldn't just need half the network, or more.
The attacker needs just 40 percent of the network? more or less, right?
It all depends on chance if reversing the block has a chance element to it.

So I ask my fellow Bitcoiners: Does it?

_{PS: Sorry for being away, I'll be on the forum from now on again!}

[Gavin Andresen]

Sure, there are a couple of tables in the back of Satoshi's bitcoin paper that calculate the chances based on what percentage of CPU work you're contributing.