Well I noticed this. When Wifi is turned on, and you move into a area that has a Wifi that is not assessable, the Droid X will not connect to the 3G at all.
I had noticed this at first when I was on College Campus and all internet connection to my phone cut off, even when I was standing outside.
I turned the wifi off to figure out what was going on, thats when I noticed I could connect to the internet again.
Problem I have with this, is if you are walking down the street using your 3G or Wifi, and enter such an area, the internet connection automatically cuts off on the phone, forcing the user to close out whatever app they are running, just to turn Wifi Off to get back the internet access.
Edit: forgot to mention the School's wifi connection is what I believe the phone is trying to connect to, but cant. So when Wifi is on, and you walk into this area, all internet connections cut off, including 3G, until the Wifi is turned off.