Hi
without code or a workaround this sort of thing is impossible but luckily for you I have a clumsy workaround which will sort this.
Say you only had two currency types and you wanted to show the price in one currency depending on what they picked. Add two identical text boxes linked to the same field. Format one as currency one and the second as currency two then use conditional formatting on them to hide when currency is blank and hide one when currency is Euro and the other when currency is pounds.
This would get very clumsy for lots of currencies so code wise you would have to write and on_after_change event on the currency selector and use the code to implement the formatting. This last bit is the bit I could not help you with unfortunately.
A simpler and wholly effective way is demonstrated in the sample form internal expense report, there a drop down box selects the currency that displays the corresponding currency symbol at the head of the monetary column. Clean, simple and effective.
Cheers Adam