Change IMEI number to obtain tablet data plan?
This would undoubtedly be a violation of T-Mobile's terms of service.  To do so, though, you would need to create a custom ROM and modify the ro.serialno system property.  Alternatively you could try to modify Build.java and pass in a different value to Build.SERIAL.  It's unclear whether that's how T-Mobile is retrieving the IMEI number though.