Java doesn't care what network you're on. Actually, it doesn't even know...
But if you're not on AT&T, you may not be able to load apps from memory card, which would mean you'll need to
- Make sure the JAD has a full address for the JAR URL
- Put the JAD on a server that serves the correct MIME type for JAD files so the phone will load it and download the JAR from the web.
Then point the phone browser to the JAD file, it'll download the JAD, then download the associated JAR that goes with it. This is how the WAP downloading of applications work, but since you need to modify the JAD file, you need to put the JAD file on your own location.