Here's my solution. I used the sowpods wordlist available here:
http://www.isc.ro/commands/lists.html (preprocessed to be all-lowercase,
though it doesn't really matter as long as the case is consistent).

It takes entirely too long to run (12 seconds on my P4/3GHz); I'll poke
at it some more if I get the time and see what's slowing it down.

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$stems = {}
$seen = {}
$cutoff = ARGV[0].to_i

IO.foreach("sowpods.txt") {|word|
  next if word.length != 8
  word.chomp!
  letters = word.split(//).sort
  alpha = letters.join
  next if $seen[alpha]
  $seen[alpha] = true
  remove = letters.uniq
  remove.each {|i|
    stem = alpha.sub(i, '')
    $stems[stem] ||= {}
    $stems[stem][i] = true
  }
}

$stem_length = {}
$stems.each {|k,v| $stems[k] = v.keys.sort}
$stems.reject! {|k,v| 
  n = v.length
  $stem_length[k] = n
  n < $cutoff
} 

results = $stems.keys.sort_by {|i| -$stem_length[i]}
results.each {|s| p [s, $stem_length[s], $stems[s].join]}