Bye the way I was talking about litecoin, not bitcoin difficulty. I never mined bitcoin.
The LTC difficulty has been rising not as fast as BTC, but I guess it will be a completely different case when the new generation of powerful scrypt ASICs are delivered.
Litecoin isn't as stable as BTC is, it can crash anytime soon. Also, if they would be mining scrypt coins, they would use a multipool instead of just LTC to maximize profits. BTC however, is a different case, there isn't much stable SHA256 alt coins and their difficulty would rise very fast.