Not every menu in Android is readily accessible, and that’s the case for the Testing menu. One way to access this interface is to type *#*#4636#*#* into the dialer. This will bring up a list of submenus. Select Phone info and you’ll be taken to an interface that gives you every imaginable bit of information about your phone’s data connection. In the drop down, you should see WCDMA Preferred; just tap that and select WCDMA Only. The phone’s radio will cycle off, then back on, and will only acquire 3G signals. You can change it back at any time.
You can skip typing things into the dialer and get right to the right menu with a free app from the Android Market. Download Anycut and use it to make a new shortcut. In the Activity list just scroll down to ‘Phone info’ and select it. You can call the shortcut whatever you like, for instance "3G Only". Now you can use this anytime you need to force 3G on. A word to the wise though, this makes EDGE signals inaccessible. So if you leave a 3G coverage area you won’t have signal. Let us know how it works if you decide to give it a try.