PHP, 401 392 382382 363 characters:
<? $h=fgets(STDIN);function s($s,$l){return str_repeat($s,$l);}$w=$h*2;$d=$h;$s="str_repeat";$w=$h*2;$d=$h/2;$b=$h;$c=" ";echo s";echo$s($c,$h/2+1)."+".s$s("-",$w)."+\n";for($i=1;$i<=$d;$i++,$a=--$b){echo secho$s($c,($h/2+1)-$i)."/".s$s($c,$w)."/".s$s($c,$i-1)."|\n";}echo"+".s$s("-",$w)."+".s$s($c,$d)."|\n";for($i=1;$i<=$h;$i++){echo"|".s$s($c,$w)."|";echo $a-->0?s$s($c,$b).($a>0?"|":"+")."\n":s$s($c,$h-$i)."/\n";}echo"+".s$s("-",$w)."+\n";
I originally did this to see how short I could manage to do this in PHP, as I knew that it would be pretty long. I'm sure it could be reduced, but not by much considering PHP doesn't have many shortcuts.
Validation:
http://codepad.viper-7.com/ftYYz9.php53
Ungolfed Version: http://codepad.viper-7.com/4D3kIA