I think that Nokia has at least investigated into making a tablet and probably have prototypes floating about the office. There's been rumours at such a frequent pace that there must be something going on at Nokia on this front.

However it could be that they have nothing at a point where they're anywhere close a release. After all, I'v read from a couple sources (Symbian Freak being one of them) that device development usually takes up to three years. And wasn't "Maemo6" being talked about already just before the N900's release?

So "not currently planned" just (probably) means that they're not ready to even think about the device's release just yet.