Books full of nonsense: Identify limericks
Python: 400 - 150 - 150 = 100

The shortest script I could come up with is that one...

import re,sys;f,e,c=re.findall,lambda l,w:f('^'+w.upper()+'  (.+)',l),lambda*v:all([a[i]==a[v[0]]for i in v]);a=[sum([[e(l,w)[0].split()for l in open('c')if e(l,w)][0]for w in f(r'\w+',v)],[])[-2:]for v in sys.stdin];n=len(a);print n==14and c(0,3,4,7)*c(1,2,5,6)*c(8,11)*c(9,12)*c(10,13)*"Sonnet"or"For a critic\nOf limerick\nWell-equipped\nIs this script.\n%s limerick!"%(n==5and c(0,1,4)and c(2,3))


...but don't even try it. It parses the provided dictionary for every word it meets, thus being very slow. Also, an error is generated whenever a word is not in the dictionary.

The code still meets the requirements though: recognizing whether the text passed via stdin is a limerick, a sonnet, or neither of those. 

With only 20 more characters, here is the optimized version:

import re,sys;f,e,c=re.findall,lambda l:f(r'^(\w+)  (.+)',l),lambda*v:all([a[i]==a[v[0]]for i in v]);d={e(l)[0][0]:e(l)[0][1].split()for l in open('c')if e(l)};a=[sum([d.get(w.upper(),[])for w in f(r'\w+',v)],[])[-2:]for v in sys.stdin];n=len(a);print n==14and c(0,3,4,7)*c(1,2,5,6)*c(8,11)*c(9,12)*c(10,13)*"Sonnet"or"For a critic\nOf limerick\nWell-equipped\nIs this script.\n%s limerick!"%(n==5and c(0,1,4)and c(2,3))


Features


able to recognize sonnets (-150)
answers to limericks with a limerick (-150)
relatively fast: only one file parsing per execution


Usage

cat poem.txt | python poem-check.py


3 different outputs are possible:


a limmerick saying the input is one if it is the case
a limmerick saying the input is not one if it is the case
"Sonnet" if the input is recognized as such


Expanded code with explanations

import re, sys

# just a shortened version of the 're.findall' function...
f = re.findall
# function used to parse a line of the dictionary
e = lambda l:f(r'^(\w+)  (.+)', l)

# create a cache of the dictionary, where each word is associated with the list of phonemes it contains
d = {e(l)[0][0]:e(l)[0][1].split(' ') for l in open('c') if e(l)}

# for each verse (line) 'v' found in the input 'sys.stdin', create a list of the phoneme it contains;
# the result array 'a' contains a list, each item of it corresponding to the last two phonemes of a verse
a = [sum([d.get(w.upper(), []) for w in f(r'\w+',v)],[])[-2:] for v in sys.stdin]

# let's store the length of 'a' in 'n'; it is actually the number of verses in the input
n = len(a)
# function used to compare the rhymes of the lines which indexes are passed as arguments
c = lambda*v:all([a[i] == a[v[0]] for i in v])

# test if the input is a sonnet, aka: it has 14 verses, verses 0, 3, 4 and 7 rhyme together, verses 1, 2, 5 and 6 rhyme together, verses 8 and 11 rhyme together, verses 9 and 12 rhyme together, verses 10 and 13 rhyme together
if n==14 and c(0,3,4,7) and c(1,2,5,6) and c(8,11) and c(9,12) and c(10,13):
    print("Sonnet")
else:
    # test if the input is a limerick, aka: it has 5 verses, verses 0, 1 and 4 rhyme together, verses 2 and 3 rhyme together
    is_limerick = n==5 and c(0,1,4) and c(2,3)
    print("For critics\nOf limericks,\nWell-equipped\nIs this script.\n%s limerick!", is_limmerick)