Following your ideas, I just applied a Reset Sync History under iSync Preferences, and worked fine for my case. (I did my backup of iCal completely but didn't have to use it in the end). I simply quited iCal, reseted Sync History, restarted iCal and synced again using iTunes and problem was solved. (Attention: I had do reconfig a few of the iTunes sync settings, such as wich mail accounts to sync between computer and iphone). I didn't have to desactivate syncing in iSync before Resetting Sync History (besides, I've noticed that it automatically desables during every sync, because I had the iSync preferences open while syncing in iTunes, although this is irrelevant for the matter), nor did I have to do any manual cleaning on iCal folder, cache files and other things.
Besides another little issue I had for some time seams to have been solved. It was nothing important but just annoying. Whenever I synced computer (mac book pro, btw) to iphone, the ruller in iphone main page (in iTunes) that shows what type of data is filling your iphones disk (and extimates free space) would allways jump up for more than a Gb at Sync start and after it get to it's normal dimension. It was like iTunes would sudenly consider a lot of new files occuping iPhone's disk (probably content form the History data, that iTunes should not consider while calculating effective occupied space).
For some moments I thought that this could probably have also solved another stupid error on syncing Notes between Mail on Mac and Notes on iPhone. (I sync notes thrugh a gmail account to work since i have this iphone, because direct sync of notes on Computer never worked fine. I would allways find all the older and oldest notes from the past, so to avoid that confusion I started syncing all notes only through Gmail). Now after this History cleaning I can't even sync at all Notes on Computer with iPhone (nothing happens or shows on iphone, ie, i can't bring the Notes on computer to the iphone). Since I was already living fine with the Gmail alternative, I didn't look anyfurther for causes or hints.
Thanks to all who helped finding this solution. Good luck for the others.