i have the phil and ted.. and hope to use the bub in the laay down feature at the back very soon!!
it is safe.. i mean DD was in it as a baby and the toddler seat it on the front (not the back)... we have set it up like this for a month now to get DD used to being up higher in the toddler upright seat.

I have used it with my sisters newborn in the back flat and no probs!