I have specified the locale as en_US (both with 
javax.servlet.jsp.jstl.fmt.locale property and with <fmt:locale> tag, neither 
makes a difference), but currencies format with a ¤ character rather than a $.

// THIS CODE DOES NOT WORK -- PRINTS ¤300.00
<fmt:locale value="en_US"/>
<fmt:formatNumber value="300" type="currency" />

// THIS CODE WORKS -- PRINTS $300.00
<% java.text.Format format = java.text.NumberFormat.getCurrencyInstance
(java.util.Locale.US); %>
<%= format.format(new Integer(300)) %>