Randy,
Differential pairs take care of differential noise (or signal) return, if
they are really closely coupled and symmetrical, hence you are in principle
correct. However, they always carry some common-mode (CMM) noise, as any
other traces, level of which depends on many factors, but it is primarily
due to the CMM noise on the chip at which the pairs originate. You do not
want that noise to cross the moat, and that is more for EMI than for SI.
For instance, a CMM choke across the moat or a pair of capacitors from each
trace to ground (same one on which the chip sits), placed near the chip,
may do the job. The pairs can pick up significant amount of CMM noise if
they are routed next to power planes. Reference them to the ground plane,
especially if they leave the board.
Neven Pischl
Bay Networks
408 495 3261