Here are some photos of the [ame="http://www.amazon.com/Britax-USA-S844900-Infant-Positioning/dp/B00422M5JQ/ref=sr_1_1?s=gateway&ie=UTF8&qid=1284997205&sr=8-1"]optional infant positioning insert from Britax[/ame]. It retails for $14.99. It should allow parents to use the Marathon for larger newborns and infants, though it probably will not accomodate preemies or tiny babies.
It is rated for use from 5-11 pounds, but that does not guarantee that it will result in a correct fit. You must still follow the instructions; the baby's shoulders should be at [or above] the level of the lowest setting for the shoulder harness slot.
With the
Huggable Images 7-pound infant doll (this is a great training doll by the way!), you can see that the doll is almost but not quite there (left). While I would consider it an acceptable fit from a general practice since the harness is quite snug and secure, the doll is still a bit short of the bottom slots so it could not yet be used for an infant this size, based on the instructions. Even so, you can see it is much better than without the insert (middle). The insert not only moves baby up closer to the harness strap slots, but also moves the baby forward a bit and helps to take up some space at the crotch and make for a better fit there, too.
Of course, real babies vary a lot in seated torso/shoulder height. Some might fit at 7 pounds, others may not. I expect that a lot more babies in the 8 to 11 pound range could fit properly with this insert. I hope to find some suitable baby models to demonstrate in the near future!
In the LEFT column: With Infant Positioning Insert, slightly different camera angles
In the MIDDLE column: NO Infant Positioning Insert
In the RIGHT column: Before (top) and after (bottom) installation