The 770 was presented on May 2005, then there was a longer pause till the N800 was presented in January 2007, and after that, the N810 came pretty quick in October 2007 (with its WiMAX cousin being announced in April 2008)...I assume one reason for the longer pause between the 770 and the N800 was that there was a platform change between these two handsets.
So - if the next tablet is OMAP3 based, as we all hope, I guess it might take a little longer till it will appear. But I still hope we'll see the next iteration in 2008...