Currently Being ModeratedNov 26, 2012 7:06 AM (in response to TB P)
How did you go about changing it to Euros? If you did it in System Preferences/Language & Text, it is a system-wide change and will apply to every app that uses that preference, including Numbers. If that is not what you did, I'd look there anyway to see if it is set to Euro (and then set it back to US Dollar if it is).
All you should have to do to change the currency symbol for a cell (or column) is to select the cell(s), go to the Cell Inspector, select Currency as the format and choose Euros.