I found the problem!
The SSID is my daughter's name. My first language is french and I use accents and the name Valérie was just not valid on the Iphone.
I changed it to Valerie and then it showed up.
...I tried single g, no security, etc... Did a lot of trouble shooting when I realized the neighbor's secure g SSID showed up from time to time, and for the first time I had an accent in the SSID...
By the way, I applied the advanced network settings as in the other thread (advanced wireless settings: beacon period: 75, RTS and Fragmentation: 2304).
Many thanks, FurryNutz!