Let me clarify something - the difference between tablet and phone.

The biggest difference is Real Time. You want to receive your phone call, right? There is no such problem on tablet, but N900 solves this for GSM, SIP and Skype in multitasking environment. Where is such experience in a world? That is a big deal - just consider the situation then video is played and call arrives. For smooth video it should have an elevated priority but phone application has it too.

The next is battery - 3G consumes battery and that phone is actually announced as constantly online. Tablet doesn't need that. Simple GSM phone or phones like iPhone or even Nexus One have NO requirement to support permanent multitasking connection to Internet. Because they allows background work only for specific applications but not any customer installed. So, N900 by requirement allows your appication communicate with world ever minute. And that eats battery!

There are another multiple differences between tablet and multitasking phone but these two are the biggest.