the password you put on your qt client is one thing and the private key is another thing!
we have 2 things here!
If you lost your password (a good password that is statiscally infeasible to brute force), your private keys will be lost (unless you have some unencrypted backups)...
Any password can be brute forced!
while private keys are something nearly impossible to brute force but with the correct algorithm they can be find out! !
Why I say nearly impossible?
Check this thread!
https://bitcointalk.org/index.php?topic=427753.0He is giving 1000BTC bounty to the guy that finds the value of k for a specific BTC address he owns
Also note that the value of k ranges from 1 to 2^256 and 2^256=115,792,089,237,316,195,423,570,985,008,690,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000
note that after 69 I've put only 0's so you can see how big the number is!
P.S. If you find the k you will get as bonus his BMW! Witch was around 100.000$