1P5: Nested boxes

Python, 204 chars

def P(n):x=raw_input();return eval('[(n+".","","-")]'+'+P(n+"| ")'*int(x))+[(n+"'",'','-')]if'0'<x<':'else[(n,x,' ')]
r=P('')
for q,t,f in r:print q+t+f*(max(len(2*x+y)for x,y,a in r)-len(2*q+t))+q[::-1]

P returns a list of triples, each of which is a line prefix/suffix (the suffix being the reverse of the prefix), some line text, and a line fill character. After calculating all the triples, they are printed using the right number of fill characters to make all the lines the same length.

Ungolfed version:

def get_lines(prefix):
  line=raw_input()
  result=[]
  if line.isdigit():
    result.append((prefix+'.', '', '-'))
    for i in xrange(int(line)):
      result += get_lines(prefix + '| ')
    result.append((prefix+"'", '', '-'))
  else:
    result.append((prefix, line, ' '))
  return result
lines=get_lines('')
width=max(2*len(prefix)+len(text) for prefix,text,fill in lines)
for prefix,text,fill in lines:
  print prefix+text+fill*(width-2*len(prefix)-len(text))+prefix[::-1]

Ruby 1.9, 174 characters

r=->l{$*<<(l*2+i=gets.chop).size;/\d/?eval('[l+?.,p=?-,p,'+'*r["| "+l],'*i.to_i+"l+?',p,p]"):[l,i,?\s]}
r[""].each_slice(3){|a,b,c|puts a+b+c*($*.max-(a*2+b).size)+a.reverse}

Somewhat similar to Keith's solution.

APL (78)

{∧/⎕D∊⍨I←⍞:{∆,('-'⍪⍵⍪'-'),∆←'.|'''/⍨1(⊃⍴⍵)1}⍕⍪/{⍵↑[2]⍨⌈/⊃∘⌽∘⍴¨∆}¨∆←∇¨⍳⍎I⋄⍉⍪I}⍬