Has any deployed this Dell model yet ? (only 1 revision of drivers available)
http://en.community.dell.com/techcenter/enterprise-client/w/wiki/11688.precision-3620-windows-7-driver-pack
Looks like the Intel USB 3.0 Root Hub driver (Windows 7 64-bit) is not being injected from SCCM 2012 R2, hence no keyboard & mouse. Luckily it has PS/2 connections and the Intel USB 3.0 Root Hub driver can be added manually.