You would declare the goods you're importing (which brings with it its own set of issues when importing high-value goods). That said, it's unclear when or if bitcoins would be 'imported'. And it's also unclear whether, for example, someone bringing in $5000 worth of gold bullion via a passenger jet would be able to come in via visa waiver (they may classify selling it as a form of work).
Anyway, the visa waiver program is for ordinary tourists. Doing something way out of the ordinary like that is just asking them to deny your visa. At the very least, apply beforehand and explain the situation to the immigration agents.
Most goods that might be imported with the intent to trade for cash have more obvious (or, in the case of, say, gold or gemstones, more universally recognized) value.
