For example, if I bought Litecoins with Bitcoins, and later sold for a gain in Bitcoins, how would I be taxed? From the article it seems I would only be taxed until I ultimately realized my gains in USD. Would I have to maintain a record of all my "altcoin" transactions in this case?
However, if that doesn't apply, you should generally owe taxes on any gain resulting from the conversion. Based on the types of properties excluded from like-kind exchange treatment, you should probably have taxable gain (or loss!) from the exchange.
According to US tax law, any accession to wealth is taxable when the gain is realized (yes, there are exceptions to timing and basis). So, if you mined a BTC and spent $1 doing it, then used that 1 BTC to purchase some LTC when the value of the BTC was $100, you gained $99 at the time you traded it for LTC. That gain is realized at the time of the exchange and taxable at that time.
There's generally no "this must become USD" rule before you owe capital gains tax.
I am not an accountant or tax lawyer.