Unfortunately with how the antenna system is designed and works on the DIR-645 class designed routers, all the antenna systems are built in for ease of use and design. I think the way of the external antenna will be limited to those routers that D-Link has already or by some way, the use of external antennas are going the way of the Floppy disks.
Ya it would be more flexible to have the ability to use external antennae however I presume one factor in all this is that the low use need of externals for the average home user perspective. Another factor could be that it has to complete with range extending products like Wireless DAPs or Power Line products too.
I noticed Apply and NG have hopped on the internal antenna router band wagon as well.
Last current live external supporting routers are the DIR-615, 655, 825, and 835.
Future routers may have external antennas for some models.