[sroecker]

It has been estimated on bitcointalk that around 1-2% have been lost. That's not really a large amount. You can still divide bitcoins into satoshis (1E-8) or even further with a protocol change in the future.

[illuminate]

Out of curiosity, does this estimate account for stolen wallets, or are these still considered viable?

[sroecker]

I did some research:

Around 0.6% of all Bitcoin are known to be lost, so with unknown losses it could be 1-2%(https://bitcointalk.org/index.php?topic=7253.120)

Excluding the Pirate scam, 2.9% have been stolen. These are still viable, some are tainted since the receiver address is known. (https://bitcointalk.org/index.php?topic=83794.0)