A BTC holder that believes the btc price would go up he would not spend his btc on a service that is currently being valued in dollars.
I see your point now. Maybe we'll think about giving both options.
I see the point too, I just think it is an absurd notion particularly as applied here.

Yet, not every BTC/USD bullish is necessarily BTC/ZEN bullish.
^^ THIS. If we are really concerned with some triangular arbitrage then we need to consider all three edges of the triangle!
On a second thought, Zencoin is a commodity, and the sale is not intended for assets speculations. So maybe we'll keep it fiat.
I think this would mostly come down to constraints of the "IPO" itself, such as how escrow would be handled etc. Ideally you should offer as broad a base of denomination as possible while still remaining convenient to everyone. ("I'll send you some apples for zencoins." is probably not logistically viable, but "btc, usd, ltc" should be trivial enough.)