How is hoarding to be prevented? If all humans used bitcoins now (Jan 2013) for most of their transactions, then a dollar would be worth about a billionth of a bitcoin (1/10 Satoshi).

If bitcoins take ten years to catch on that fully, and population rises only a little while worldwide productivity doubles, then you could buy 5 candy bars for a Satoshi (1/10^8 BTC).

A house today, starter home might be worth 10 000 bitcoins or so. Paying a 30 year loan payment in Bitcoins would require a payment of a billion dollars a month at some point.

So I don't see how you could have viable loans in bitcoins until the system has settled down at least 30 years from now.

And if you have 1 bitcoin today, it could become worth 100 million dollars purchasing power in 10 to 20 years. One inexpensive coin worth an hour's labor. 

So the system has an enormous potential reward for hoarding--far beyond working for a living, or any remotely normal successful investment.

How is hoarding to be prevented?

And I think this system is even more unsound than fiat money, albeit in a very different direction. But it IS likely to lead to drastically improved money in the long run.
Key points in bold.

Hoarding doesn't need to be prevented. At least, not yet. See this answer for a solid argument: How does hoarding hurt Bitcoin?

Bitcoin attracts hoarding by design. To change that would turn it into something else.

As Bitcoin matures, I expect that it will remain a problem that speculative trading brings  instability to the market, similar to the gold market. If you put your retirement savings into gold, you'll likely do quite nicely, but there's also a significant chance you'll lose most of it in a price crash. It's not a reliable store of value. Even if you have several years you can wait for the value to come back up, you may still lose out.

Even though bitcoins are more efficient to trade than gold and cheaper to store, it's subject to the same volatility when traders manipulate it. The more popularity Bitcoin has, the bigger the traders will be that take an interest in it, like they do in gold. This could mean that when Bitcoin is mature we still see the same level of volatility that we see today with Bitcoin (and gold).

To me, this says that Bitcoin is great for efficient online transactions, but in the long run (once the market has reached its saturation point) won't a completely reliable store of value. For that, a diversified portfolio of productive investments is better.

If that becomes common knowledge, then people will eventually accept the volatility as a necessary risk in the use of Bitcoin. After all, on average it will still almost certainly cost less than credit card transactions and traditional international money transfers. Secondly, risk-reduction services (ie insurance) tend to fill gaps in the market like that for those who don't want to take the risk.

In the meantime, Bitcoin is doing a great job, but it doesn't mean something better won't come along.