After receiving my Droid 3 a few days ago, I fought with it on the syncing/push issue. I read the numerous forum posts, tried lots of things (yes, I cleared the application cache and everything else under the sun). Eventually I even did a hard reset of the entire device. At some point I noticed (not sure how) that when I was switching off of WiFi, I suddenly got an e-mail notification. Sure enough, when I tested with WiFi off, push worked great on 3G, but with WiFi on, only a manual refresh would get new e-mail on GMail.
Long story short, it is a firewall issue. I traced the traffic over WiFi and the Android tried to open up a port to an Internet server on TCP/5228. If it is blocked (as it was in my case) then push won't work. If it is unblocked, then voila, Push works on WiFi.
So, if anyone is out there bang their head on the wall as I was - if you have access to the firewall of your WiFi network, simply allow TCP/5228 outbound to the Internet and things will likely clear up. Why they don't do this on 80 or 443 is beyond me, but at least I solved it in my case.
Good Luck to all - love my Droid 3!