How does one document Bitcoin investments for tax and other legal issues? If Bitcoins are used as a store of value or investment vehicle, what legal documents should the investor require/keep? To date, no jurisdiction has special laws concerning cryptocurrencies, but investors may want to implemented best-practices to avoid later hassle.

Personally, I'd expect a receipt for any fiat-BTC transaction indicating


full name and address of the other party or exchange,
amount in BTC and fiat currency, and
any fees and taxes incurred.


Is this reasonable? Do larger exchanges, Mt.Gox or Bitcoin-central, issue a printable document with this information?
What legal documents to keep?  That's really a question for your accountant and/or attorney. One could reasonably expect to need about the same level of documentation as would be required for an investment in stamps or coins. 

Most likely the exchanges comply with AML (Anti-Money-Laundering) statutes in their own jurisdictions, but the onus is probably on the individual to obtain documentation of all transactions.