> The only way I can think of to get around it is something like:
> 
> <xsl:choose>
> 	<xsl:when test="string(number(discount))='NaN'">
> 		Discount: <xsl:value-of 
> select="format-number(0, $currencyformat)"/>
> 	</xsl:when>
> 	<xsl:otherwise>
> 		Discount: <xsl:value-of 
> select="format-number(discount, $currencyformat)"/>
> 	</xsl:otherwise>
> </xsl:choose>
> 
> Seem awfully long-winded. Anyone got any ideas?

As Andrew says, long-windedness in XSLT 1.0 is a fact of life. But there are
some coding tricks available when it gets out of hand. For example

format-number(concat('0', discount), $currencyformat)

Michael Kay
http://www.saxonica.com/