Jim, do you do that over Bluetooth or with the cable? I do the same with the cable, but could not get it to work with Bluetooth.
I too have seen on occasions it copies all photos instead of all the new ones. I think it only happens after an iOS upgrade on the phone, must cause Picasa to think it's a new device, so it imports everything. I don't use Picasa to import though, I use the built-in Windows photo import feature, but I use Picasa to view/edit pictures after though.