It is estimated that the Oyapock Bridge between France and Brazil will be finished in 2012. I have already been to Oyapock and ever since I dreamt to do a road trip starting in French Guyana. Looking on Google Maps, the roads from Oyapock into Brazil seem to end in the Amazon rain forest and in Macapa. While in Oyapock 5 years ago, I saw quite some cars driving around. This made me wonder if there are onward journeys possible from Macapa.
Agreed, Macapá is in the most deserted state of Brazil. Being right by the mouth of the Amazon river, you run into the same problem that Oyapack suffered until the bridge - a giant expanse of water.
However, taxis for R$30 can take you 25km south-west to the town of Santana. From there, you can can travel by boat across the river to Belém(24 hours), Santarem (40 hours, hammock R$ 125, cabin R$ 250) and Manaus(5 days) - plus some other smaller places. From these cities you can easily get buses to travel further south.
I reckon one of my dream trips is to do this to Manaus, then to take a 6 day boat ride up the Amazon to Leticia in Colombia. It sounds amazing!
I just came across a video blog of some one doing that trip in oposite direction. So far it seems feasible by bike.