This really isn't a hint -- it's a warning based on an experience I had with Address Book during a sync. I run Tiger and, during a sync, I was adding entries to my Address Book. I know that iCal used to not let you make changes while a sync was happening.
I assumed (yeah, I know about assuming) that, since Address Book didn't complain, it was safe. Well, when the sync was done, all I had in my Address Book were the two addresses I had added during the sync! Thankfully, I have .Mac and was able to recover my contacts from one of my registered Macs. If I didn't have that to fall back on, though, I would have lost all my contacts. I also noticed that iCal no longer compains if you open it during a sync. So be careful...
[robg adds: I haven't confirmed this one, but it seems like a reasonable caution to be aware of when syncing!]

