1) Is Bitcoin-Denominated Debt Enforceable?
In other words, say I lend someone X BTC (unsecured), and get them to sign a promissory note for the X BTC loan, etc... etc... Now, assume they didn't pay me back. Could I theoretically pursue them for this debt, and potentially take them to court? If so, would I receive only the USD value of BTC lent, or is there a way of writing a contract such that they would be obligated to provide me with the actual X BTC, no matter how much it is worth in USD terms, now?
Yes, there is. I've dealt with these agreements before. It's not as simple as just substituting BTC for USD in the text of the agreement, but it isn't impossible either.
2) Do normal usury laws apply to BTC?
Self explanatory.
It would depend heavily upon the state. Usury is a question of state law in the US.
3) Do I have to report income tax on interest earned from BTC loans?
Or do I only have to pay such tax when/if I exchange that newly earned BTC into USD?
I honestly don't know the answer to this one. The tax professionals I work with typically reason by analogy when dealing with Bitcoin income. If there is some other in-kind investment out there that pays dividends in-kind, then they would probably use its tax treatment as precedent.