Great ideas! Here's my implementation of the first idea:

#!/usr/bin/perl -w
# example usage: rfc 1459     (gets the IRC RFC)

use strict;
use LWP::Simple qw(get);

die("usage: rfc <rfc#>\n") unless defined($ARGV[0]);

my $rfcnum=$ARGV[0];
my $base="http://www.rfc.net/rfc";
my $rfcdir="$ENV{HOME}/rfcs";
my $rfcfile="${rfcdir}/rfc${rfcnum}.txt";
my $rfc;

if(-e $rfcfile) {
    open(RFC,"<$rfcfile") or die("open($rfcfile): $!\n");
    local $/ = undef;
    $rfc=<RFC>;
    close(RFC);
} else {
    $rfc=get($base.$rfcnum.".txt");
    open(RFC,">$rfcfile") or die("open($rfcfile): $!\n");
    print RFC "$rfc";
    close(RFC);
}

print "$rfc";
__END__
[download]

(Update: Just when I thought I had it nailed, better ideas crop up. Now reading docs to retool to merlyn's idea below. (tho rfc.net doesn't appear to return the proper headers that support mirror()))

--
Me spell chucker work grate. Need grandma chicken