while(arraySize--) array.push(value);

no initialization (that i know of)

Update

Since ever posting this answer 4 years ago, people seem to keep coming back here for this answer. For benchmarking purposes I made a JSPerf with some different solutions.

The solution above here isn't the quickest, although it's short. To stick to the same short style, but with a better performance:

while(size--) array[size] = value;

Update Feb 2016
Updated the JSPerf with a new revision with more testcases.

If performance doesn't matter and you want a one-liner:

var value = 1234, // can be replaced by a fixed value

    size  = 1000, // can be replaced by a fixed value

    array = Array.apply(null,{length: size}).map(function() { return value; });

A more performant solution (in one, dirty, line):
Be aware: this replaces existsing value, size and i variables in the scope

for(var i = 0, value = 1234, size = 1000, array = new Array(1000); i < size; i++) array[i] = value;