Well, if I'm allowed to generate a list of strings rather than a list of lists, then I'll kick off the golfing with a strict compliant one in 70 characters (inside of P):

#!/usr/bin/perl -w
use strict;

sub P {
my($i)=@_;push@_,grep!/[^1-$i]/,map"$i$_",P($_[0]-$i)and$i--while$i;@_
}

for(P(5)){
    print "$_\n";
}
__END__
# output is

5
41
32
311
221
2111
11111

[download]

If you do insist on a list of lists, we can tack on the following 16 character wrapper sub to call instead of P:

sub W{map{[split//]}&P}
[download]

Update: In response to my major oversight that tilly's followup points out, all I can say is:
"Rats! Last time I go golfing in the middle of the night!"
:-)

Update2:Ok, in the light of day, here's a recursive one in 82 chars (ignoring unnecessary whitespace) that returns a list of lists:

sub P{
 my$i=my$n=pop;
 push@_,grep{!grep{$_>$i}@$_}map{[$i,@$_]}P($n-$i)while$i-->1;
 [$n],@_
}
[download]