Well, you can sort of. If you hold the unit upside down and turn down the volume, you're ear is right around the speaker and your mouth is close to the mic. Of course, it was intended to be used as a speaker phone, so while this would work, it's not ideal. For example, the mic is likely to be super sensitive and any movement (ie. you brush the surface of the device) will overload it with sound. Most devices won't transmit the noise, so you'll get breaks in the conversation. In other words, speaker phones were designed to be stationary.

I sometimes use my N810 as a hold-to-ear phone and use a similar tactic, but it's far from ideal, and a real handset is far better for the task. Still, it works in a pinch. Though any movement (repositioning of hand, changing position, etc) will cause a ton of noise or breaks in the conversation to the listener. Thus, I have to hold very still to make it work properly.

I think a headset is better. Though if the Tab had a built-in phone speaker, I wouldn't mind holding it to my head. Most people would feel stupid doing this, but being uber confident, it simply wouldn't bother me.