quoted_printable_decode still doesn't handle 'soft' line breaks or padding correctly.  If you want to see it fail, try running it on the encoded string given in RFC-1521 section 5.1 rule #5.  To save you the trouble this is what code to do that would look like.

<?
# the extra spaces and tabs after the '='s where added by
# me to demonstrate that they aren't handled properly either
# see rule 3 in the same section of the RFC
$string = "Now's the time =   \r\n";
$string .= "for all folk to come=\t \t \r\n";
$string .= " to the aid of their country.";
echo quoted_printable_decode($string);
?>

The = characters in the out put are not supposed to be there.

To save you some time, here is the patch I use to fix this problem.  It's a patch against 3.0.11.  But this function hasn't changed much (if at all) since then.

--- php-3.0.11/functions/quot_print.c.orig  Wed Jun 16 06:34:22 1999
+++ php-3.0.11/functions/quot_print.c Wed Jul 28 12:03:19 1999
@@ -94,11 +94,21 @@
                   + _php3_hex2int((int)str[i+2]);
        i += 3;
      }
-     else if ( str[i] == 13 )
-     {
-       i++;
-     }
-     else
+      else if ( str[i] == '=' )
+      {
+        if( str[i+1] == 10 || str[i+1] == 13 )
+          i+=2;
+        else if( str[i+1] == 13 && str[i+2] == 10 )
+          i+=3;
+      }
+      else if ( (str[i] == 9 || str[i] == 32) && str[i+1] == 13 )
+      {
+              while ( str[j] == 9 || str[j] == 32 )
+                      j--;
+              j++;
+              str[j++] = str[i++];
+      }
+      else
      {
        str[j++] = str[i++];
      }

I've also checked 4.0b3 and the bug exists there as well.