A trawl through Google identified the following as helpful ... but this is by no means an exhaustive list!!
http://www.franceonfoot.com - overview of long-distance walking trails in France
http://www.ffrandonnee.fr - French overview of long-distance trails
http://weather.yahoo.com/regional/FRXX.html - online weather forecasts for up to 10 days!
http://homepage.mac.com/billtomlinson/Alpstrek.html - an account from 2006 of the GR5 trail http://www.grfive.com/index.htm - another account including information on the GR52
http://www.gites-refuges.com/ - details of all the mountain refuges in France
http://www.refuges.info/ - further details of mountain refuges
http://www.space-between.co.uk/ - an adventure holiday company run by a British couple with walking / activity weeks and events for families and small groups in the Mercantour National park