Memoized recursive functions. How to make them fool-proof?

Question

Memoized functions are functions which remember values they have found. Look in the doc center for some background on this in Mathematica, if necessary.

Suppose you have

Answer 1

A slight modification on Leonid's code. I guess I should post it as a comment, but the lack of comment formatting makes it impossible.

Self adaptive Recursion Limit

Clear[f];
$RecursionLimit = 20;
Module[{ff},
 ff[0] = ff[1] = 1;
 ff[x_] := 
  ff[x] = Block[{$RecursionLimit = $RecursionLimit + 2},  ff[x - 1] + ff[x - 2]];
 f[x_Integer] := f[x] = ff[x]]

f[30]
(*
-> 1346269
*)

$RecursionLimit
(*
-> 20
*)

Edit

Trying to set $RecursionLimit sparsely:

Clear[f];
$RecursionLimit = 20;
Module[{ff}, ff[0] = ff[1] = 1;
 ff[x_] := ff[x] =
   Block[{$RecursionLimit =
      If[Length@Stack[] > $RecursionLimit - 5, $RecursionLimit + 5, $RecursionLimit]}, 
       ff[x - 1] + ff[x - 2]];
 f[x_Integer] := f[x] = ff[x]]  

Not sure how useful it is ...