You should check to see if your old SOA is still showing themselves as authoritative for your domain. If they are, then anyone who uses their nameservers will still get the old record(s) for your domain.
If they are still showing themselves as authoritative (which I think is the case) you will want to contact them to have them drop your domain from their server(s), or at least get your host records updated to point to your new servers.
Yes, that does seem to be the case.
Nicolas, if you make the query directly from the nfrance.com servers you can see what we mean:
$ host -a microlinux.fr ns.nfrance.com Trying "microlinux.fr" Using domain server: Name: ns.nfrance.com Address: 80.247.224.77#53 Aliases:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 2088 ;; flags: qr aa rd; QUERY: 1, ANSWER: 6, AUTHORITY: 0, ADDITIONAL: 7
;; QUESTION SECTION: ;microlinux.fr. IN ANY
;; ANSWER SECTION: microlinux.fr. 9600 IN SOA romeo.hebergement-discount.com. dns.nfrance.com. 2012081401 43200 1800 864000 9600 microlinux.fr. 9600 IN NS ns2.slconseil.com. microlinux.fr. 9600 IN NS ns1.slconseil.com. microlinux.fr. 9600 IN NS ns3.slconseil.com. microlinux.fr. 9600 IN MX 10 mx1.nfrance.com. microlinux.fr. 9600 IN A 80.247.228.60
;; ADDITIONAL SECTION: ns1.slconseil.com. 432000 IN A 80.247.224.77 ns1.slconseil.com. 432000 IN AAAA 2a00:1a60:22e0:4d00::1 ns2.slconseil.com. 432000 IN A 80.247.228.129 ns2.slconseil.com. 432000 IN AAAA 2a00:1a60:21e4:8100::1 ns3.slconseil.com. 432000 IN A 81.25.194.8 mx1.nfrance.com. 60 IN A 80.247.229.29 mx1.nfrance.com. 60 IN A 80.247.228.29
So nfrance.com still claim SOA on your domain. At least they have a relatively short TTL on it of 9600s so things will correct themselves quite quickly once they drop the domain.
P.