i don't know why nokia bug when i swapon a 32Mo directly but i've found a solution to fix this problem. I have created a 1Mo swap, swapon on this 1Mo, after swapon on a 32Mo swap file and swapoff the 1Mo swap file and it's working well...
for example, my free mem is now :
free
total used free shared buffers
Mem: 61828 60412 1416 0 16
Swap: 32760 23896 8864
Total: 94588 84308 10280

I will buy a 1Go RS-MMC and test with larger swap after...
i will just add this to the startup i think :
swapon /media/mmc1/1Mo
swapon /media/mmc1/swap
swapoff /media/mmc1/1Mo
with some tests i think...