Find Possible Word Rectangles
Python, 232 chars
x,y=input()
H=[]
P={}
for w in open('words.txt'):
 l=len(w)-2
 if l==x:H+=[w]
 if l==y:
  for i in range(y+1):P[w[:i]]=1
def B(s):
 if(x+2)*y-len(s):[B(s+w)for w in H if all((s+w)[i::x+2]in P for i in range(x))]
 else:print s
B('')

It can only handle up to 6x6 in the 1/2 hour limit, though.