Usando el Ajax.inPlaceEditor

He creado el taglib que viene en el libro "Grails - The definitive guide" :

class ProductTagLib {

 def editInPlace = { attrs, body -> 
  def rows = attrs.rows ? attrs.rows : 0 
  def cols = attrs.cols ? attrs.cols : 0 
  def id = attrs.remove('id') 
  out << "<span id='productTags${id}'>" 
         out << body()
  out << "</span>" 
  out << "<script type='text/javascript'>" 
  out << "new Ajax.InPlaceEditor('productTags${id}', '" 
  out << createLink(attrs) 
  out << "',{"  
     if(rows) 
      out << "rows:${rows}," 
  if(cols) 
      out << "cols:${cols}," 
     if(attrs.paramName) { 
          out <<  "callback: " + 
            "function(form, value) { " + 
                     "return '${attrs.paramName}=' + encodeURIComponent(value) " + 
                     "}" 
            } 

     out << "});" 
     out << "</script>" 
 } 

}

En el show.jsp he usado el tag (usando también el toString con delimitador de la entrada anterior:

<g:editInPlace id="productTags${productInstance.id}" url="[action:'updateTags',id:productInstance.id]" 
rows="1" cols= "40" paramName="tags">${productInstance?.tags?.toString(", ").encodeAsHTML()}</g:editInPlace>

y simplemente queda crear el método en el controller correspondiente

 def updateTags = {
  def productInstance = Product.get( params.id )
  productInstance.parseTags(params.tags,",")
  render productInstance.tags.toString(", ")
 }