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).
bji has it one hundred percent correct. If the value of Bitcoins is to rise, it will be due to increased adoption, and more likely, widespread adoption. Remember, the increasing value today is due to speculation that they will become a useful vehicle for commerce, not because they are currently a useful vehicle for commerce.
Widespread adoption will result in increased value, but as bji says, it will not likely result in a decreased rate of loss. In the far future, widespread use might mean hundreds of millions, or billions of users. Regardless of increased value, due to diminishing numbers in circulation, the increased user base will cause any one wallet to likely have a relatively constant average value, based on external economic factors. This wallet value will be no different than average values of checking accounts and wallets in use today. Increased valuations of Bitcoins is unlikely to mean that any one wallet in existence is protected at a security level greater than a wallet today.
The widespread use, if it comes to fruition, will mean exposure all across the world at a high density level, and even the Solar System, if we are talking about the far future. Bitcoins will be subject to all the same accidents that can befall physical objects today, such as natural disasters (how many Bitcoins were lost in the recent Japanese tsunami?), but Bitcoins are also subject to loss via mechanisms that usually aren't an issue with normal physical objects: hardware failure of the hosting device, software viruses, accidental deletion, malicious deletion, and so on.
I contend that the most likely vector for Bitcoins to gain widespread use will be if they supplant the unstable currencies of the Third World, usable on smartphones, allowing near instant commerce between the citizens of the Third World, across state lines. This is the exact scenario which creates a situation ripe for loss, but is also likely the only real hope Bitcoins have for mass adoption.
If Bitcoins fall out of favor, either due to competing currencies, the increasing uncertainty that has been described here, or actually fail to gain widespread use, then the speculation which attaches an increasing value to them today will not guarantee a high valuation for them in the future,.