When a user loses his wallet, it has the effect of removing money out of circulation. Lost bitcoins still remain in the block chain just like any other bitcoins. However, lost bitcoins remain dormant forever because there is no way for anybody to find the private key(s) that would allow them to be spent again. Because of the law of supply and demand, when fewer bitcoins are available, the ones that are left will be in higher demand and increase in value to compensate.
Yes, the Bitcoins will remain in the block chain forever and they can't be withdrawn by anyone else than the person who owns the private keys of the wallet where they are stored. Cracking of the Bitcoin wallet private keys is near to impossible in this date, but it might be a concern for the coming time.
Bitcoins are very very safe in those wallets where they are storied, and even the most professional hackers and crackers aren't able to unlock it without having the private keys.
It's also true that when the lower supply of the Bitcoins can result in its value to increase, but that isn't a guarantee though. And, the coins supply won't be affected for just few of coins stored in a wallet forever.