Your signature says you have 10.5.5 - is this correct? Leopard is not compliant with iCloud. If you've been able to emable PhotoStream in iPhoto that does suggest you have Lion.
MobileMe will be terminated at the end of June. Before then you should sign out in System Preferences>MobileMe - before doing so you should turn off any syncing you have enabled in the Sync and iDisk panes to prevent possible problems.
Yes Roger thanks, I have Lion.
But till Mobile me will be terminated there will be problem of idenification. For example I saw that it was useful to deselect all sync by itunes for my iPhone because there is a conflict with sync by iCloud.
Also in my mailbox I had an .Mac account wich has been doubled by an ICloud mail box and now only my iCloud mail box receives mail. So I think I can suppress my .Mac mail box which doesn't connect anymore.
Mobile me used to be so simple. I wish they had change once iCloud was really completed.
So I have a dropbox now to do what mobile me used to do and iCloud is not able to do anymore.