apologies OP for threadjacking, hope you don't mind a little bump )
It was more about the not having money part, not the bitcoins that caused it. Customs expects you to have money for your trip, cause if you don't they'll assume you are here to work and not allow you entrance.
true, don't know about many other countries, few that i visited didn't care much coming from US. but US is anal about these sort of things (while our borders almost wide open), they need documented assurances that you won't stay after your visa expires and that you have enough money for the trip or that someone can support you here with their financials