At NetSuite, we are discussing how we would implement the ability to enter transactions in different currencies for the same customer or vendor.  (For which the current workaround is to create multiple customer or vendor records.)
There a number of challenges involved in this feature and we would like feedback on some specific questions about how this need arises in your business, and how you would expect it to impact other features.

1. When you say you sell to or purchase from a company in more than one currency, exactly what is the real world business case?   In other words, are you transacting with multiple subsidiaries of a multi-country organization, each with their own base currency?   Or is it really the same legal entity transacting with your company in multiple currencies?