Good to know

E. Jardin & C. Schoch

Selected websites on France US mobility.

Before leaving
The Toile des Biologistes defines itself as "the network of Francophone researchers in life sciences." This association publishes the absolute must-have: "Guide to postdocs abroad" (downloadable).
Postdoc offers that you can select by discipline and by geographic area with an information sheet on the job offer. 
Mobility grants. 
Information on immigration visas for researchers 
Average postdoc salaries in the US by large cities.
NB: a mailing list.

Coming back 
The science and technology mission of the French embassy in the United States announces recruitment fairs that are held in the United States in Boston for the "MIT European Career Fair" and in San Francisco for the " International Career Fair." Also note the "Young Entrepreneurs Initiative" to encourage companies start ups upon return to France.
421 are listed.

A list of job websites.
The Biodocs website (Association of young researchers in biology)