Hi os008. I think your understanding is generally correct. Individuals can, have in the past, and will in the future (there are some posts that reference people deleting their wallets) lose their wallets, which will effectively destroy those bitcoins forever. The lose of those bitcoins will likely have no immediate effect on the value of everyone else's bitcoins, but will reduce the supply of bitcoins. Consider a central bank that can print money, increase the money supply relative to demand, and ultimately cause the money to lose value (in purchasing power and in trading value against other currencies). Reducing the supply of money relative to demand will have the opposite effect -- increasing the value of the currency.

My guess is that (excluding a mass loss of bitcoins due to a virus or some other disaster) there is likely to be a small percentage of total bitcoins lost, which will have some small but meaningful effect on the value of bitcoins.