> <xsl:apply-templates select="substring-after(@rdf:about,'#')"> 
> </xsl:apply-templates>

You can only apply templates to nodes, and substring-after produces a
string, not a node. 
> 
> Can anyone guide me how can I write the result of this 
> substring-after thing into an XML file.

Use xsl:value-of

> 
> Btw, I tried using following code: 
> <xsl:for-each select="@rdf:about"> 
> <xsl:value-of select="substring-after(@about,'#')"/> 
> </xsl:for-each> 
> 

Attributes don't have attributes. Your code is trying to display (part of)
the @about attribute of the @rdf:about attribute, and there isn't one.

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