The base currency for an US index is always going to be in US dollar (someone correct me if I'm wrong). However you can purchase an iShares S&P 500 ETF that is traded in GB pounds on the LSE or Euro on Euronext. Changes in the Dollar GB or Dollar Euro exchange rate get reflected in the ETF price on the relevant exchange.
bogle- your bang on the money. its the same as a global equity fund that has equities domiciled and denominated in many different currencies but the unit price is in euros.
Hope I'm being clear. The us stocks are translated to a euro value and a nav (what the price of the fund is typically bought & sod for) is calculated