Update: i added petral's and impossiblerobot's routines to the comparison...

although my solution was complex and used regex mojo, chipmunk's is by far the faster of the two. impossiblerobot's first is fastest of all, becase there's no real thinking involved. consider

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

my $s = '00a0c801adc6';

timethese(100000,{
    particle => sub {
        my $str = $s;
        $str =~ s/([\w]{2})(?!$)/$1:/gio;
    },

    chipmunk => sub {
        my $str = $s;
        $str = join(':', $str =~ /../g);
    },

    petral => sub {
        my $str = $s;
        $str = join(':', split /(?=(?:..)+$)/,$str);
    },

    robot1 => sub {
        my $str = $s;
        $str = join(':', unpack 'a2a2a2a2a2a2', $str);
    },

    robot2 => sub {
        my $str = $s;
        $str = join(':', unpack('a2' x (length($str)/2), $str));
    },

});
[download]

which results, on my 450MHz, 768MB ram, win98 box, in:

C:\WINDOWS\Desktop>perl test_mac.pl
Benchmark: timing 100000 iterations of chipmunk, particle, petral, rob
+ot1, robot2...
  chipmunk:  3 wallclock secs ( 2.52 usr +  0.00 sys = 2.52 CPU) @ 396
+82.54/s (n=100000)
  particle:  6 wallclock secs ( 6.48 usr +  0.00 sys = 6.48 CPU) @ 154
+32.10/s (n=100000)
    petral:  5 wallclock secs ( 6.05 usr +  0.00 sys = 6.05 CPU) @ 165
+28.93/s (n=100000)
    robot1:  2 wallclock secs ( 1.27 usr +  0.00 sys = 1.27 CPU) @ 787
+40.16/s (n=100000)
    robot2:  3 wallclock secs ( 3.02 usr +  0.00 sys = 3.02 CPU) @ 331
+12.58/s (n=100000)
[download]

this goes to show you just how much more time regexes can add to your algorithm.

~Particle