agreed, clearly the discontinuity in the demand is based on the non-price costs, which is why people like the idea of easy micropayments so much. But the psychological difference between free and not-free is pretty significant.

Exactly. Even with micropayments, I would still have to think about it, and I don't like having to think. This is part of the reason why the most popular micropayment system on the web is Google AdSense.

On the other hand, I regularly send text messages on my phone without much though given to the crazy $0.15 per-message fee.

same with "registering" in the first place, even if it is free. Just knowing that you have to make a decision whether or not you can use your real email address, the nagging question of what they're doing with the information, etc...