A little Reflectoring into the XMLSerializer source shows that in .NET 1.1 it caches the temp assembly using both the type and default namespace as the key. However, 1.0 cached only on the type, and if you specified a namespace it ignored the cache. And it looks like even in 1.1 the other constructor overloads ignore the cache. Good to know.

And the reflectoring shows just that: (Note the Monitor.Enter [lock()] around their cache)

public XmlSerializer(Type type, string defaultNamespace){ 
  XmlReflectionImporter importer1;
  TempAssemblyCache cache1;
  this.events = 0;
  base..ctor();
  this.events.sender = this;
  this.tempAssembly = XmlSerializer.cache[defaultNamespace, type];
  if (this.tempAssembly == null){
    cache1 = XmlSerializer.cache;
    Monitor.Enter(XmlSerializer.cache);
    try{
       this.tempAssembly = XmlSerializer.cache[defaultNamespace, type];
       if (this.tempAssembly == null){
          importer1 = new XmlReflectionImporter(defaultNamespace);
          this.tempAssembly = XmlSerializer.GenerateTempAssembly(importer1.ImportTypeMapping(type));
          XmlSerializer.cache.Add(defaultNamespace, type, this.tempAssembly);
     }
}
finally{  Monitor.Exit(cache1); } } }