So right now the emphasis is on making the platform support a phone. Because everyone knows Maemo can run a tablet,. And right now the new device is one with telephony, so they can get that bit working.
There is absolutely nothing stopping those who want a tablet using one. And personally, I think it's [extremely likely that step 5 will include both sorts of devices. Indeed, possibly lots of devices with a range of hardware specs for different target audiences much as symbian does.
Let Nokia develop its platform first.