This week's Autocar has a two pager on the new A3 Convertible. The image is just a mock up of an S3 with the roof lopped off, but they do go into detail about it.
Aside from the current engines, there will also be an entry level 1.2 TFSI pumping out 140hp (or something like that). This will become the entry level engine for the entire A3 range. It is due to be unveiled in September and on sale by March 2008. They say that it will be pretty similar in shape to the A3 except the bonnet will be reshaped to offer better pedestrian safety.
It will be based on the Eos platform and be a full four seater, with an A4 style fabric roof with a glass rear screen which retracts into an area behind the rear seats.
It also appears to have the A4 style chrome A pillars.