17" is already straining the limits of portability, I think. The only useful portability of a 20" laptop would be in moving it from room to room, which you could do with a 20" iMac instead. If you actually want to carry a laptop in a laptop bag and take it to school, work, etc., I think a 20" is just too big...
Edit: Actually, I could see a need for a 20" laptop for exclusive home use. An iMac can be moved from desk to desk (or table to table), but then you also have to lug the mouse and keyboard with it. And what if you want to work kicking back in a chair or even from bed? So yes, I think there is a place for a 20". But that being said, I think Apple will probably determine that the market for a 20" laptop isn't large enough yet.