According to Jekyll's post which has a link to a pdf outlining some of these new models, it looks like Synology will use infinband to connect the expansion units.
Rear pictures of the units also show the units have either an infiniband connection or a SAS connection.
The dual NIC ports could either be Gigabit or 10GBASE-T. However if they are only Gigabit then it wont be possible to reach the transfer speeds that Synology are quoting, given this either Synology are quoting internal transfer speeds between the expansion unit and the DiskStation, or the Dual NIC ports on the rear are in fact 10GBASE-T.
Time will tell, but it looks as if Synology are making a good attempt at an enterprise level product.
"The NASman - Storage for your digital future"
Synology Model: DS1511+, CS407e
Firmware: 1358, 1285
HDDs: 4 x Samsung HD204UI 2TB
HDDs: 4 x Samsung HD501LJ 500GB
DMAs: 1 x XBMC,1xWDTV Live