I am not so sure.  If bitcoin becomes widespread in use, then I would think that there would be a spreading around of bitcoins more than anything else.  So if right now there are 10000 people with an average of 200 bitcoins each (just to pick some numbers), then if bitcoin becomes very popular then there might be 1000000 people with on average 2 bitcoins each.  Now there are many, many more opportunities for people to lose bitcoins.  We can see that even today with a high value of ~$20 per bitcoin, some people lose them due to carelessness (the recent heist because a person didn't bother to encrypt their wallet file comes to mind), and even web sites that were pretty much printing money (mtgox) don't do a good job of securing their coins and (possibly - hopefully!) killed their goose that was laying golden eggs.

So clearly just because bitcoins are valuable, it doesn't mean that there won't be careless people who lose them.  And if you spread the bitcoins around, there will be less value to lose per individual 'wallet', but many more wallets.  I personally think it's kind of a wash, and I would expect a constant rate of loss that actually exceeds that of paper money since digital currency like bitcoins is so much easier to destroy (with a single keystroke you can destroy a large quantity of bitcoins instantly; with paper money you'd probably have to throw a big suitcase into the ocean or something, which takes alot more effort).