[kh99]

In your plugin code you can use $threadinfo['threadid']. If you want to use it in the SHOWTHHEAD template it's already registered, and you can use {threadinfo.threadid}. You would only need to register it if you're rendering a custom template, in which case you'd use register() (not preRegister). If you really do want to preRegister to one of the other existing vb templates (e.g. header, headinclude, footer), then you might want to use hook parse_templates (but it really depends on exactly what you're trying to do).