Hiya, I'm sure I'm just looking at this upside down, but here goes:

I want to do something like this:

<xsl:template name="makeSegLabel">
<xsl:variable name="ana" select="tokenize(@ana, '(\s+)')"/>
<xsl:for-each select="$ana">
  <xsl:value-of select="."/><xsl:text>:</xsl:text>
  <xsl:value-of select="//category[@id = .]/catDesc"/>
</xsl:for-each>
</xsl:template>

Where I want to tokenize the ana attribute on the element which called
this template, and then for each whitespace separated value, I want to
put it out, and then go get a description of what that value means
from elsewhere in the document.

However, I'm rightly told that I cannot select a node here because the
context is an atomic value.

I'm sure I'm being stupid and there is a better way to do this.

No, this is the right way - you just need to maintain a pointer the
content node (or root) outside of the tokenize, for example:

<xsl:template name="makeSegLabel">
 <xsl:variable name="ana" select="tokenize(@ana, '(\s+)')"/>
 <xsl:variable name="this" select="."/>
 <xsl:for-each select="$ana">
   <xsl:value-of select="$this"/>

<xsl:variable name="ana" select="tokenize(@ana, '(\s+)')"/>
<xsl:value-of select="for $x in $ana return string-join(concat($x,':',
key('cat-by-id', $x)/catDesc), ', ')"/>

cheers
andrew