howto secure older keys after the recent attacks

[reynt0]

On Fri, 2009-09-11 Christoph Anton Mitterer wrote:
  . . .
> sell bad devices that produce poor entropy thus rendering
> our (symmetric and asymmetric) keys, signatures etc. "useless".
  . . .

Just out of curiousity, about how "poor" entropy might make 
it easy to break encryption:  Is it necessary for an attacker 
to know pre-attack that a specific targeted file has been
encrypted or etc using poor entropy?  Or is the weakness one
which can efficiently be exploited en mass, by attacking all
files in a batch of files and just being successful against
any of the files which happened to be encrypted using poor
entropy?