After root, use a file explorer.
I use the ES File Explorer downloaded from the market:
- Launch it
- Goto "Settings> File Settings". Be sure that Show hidden files is checked
- Also be sure is checked "Settings > Root Explorer"
- In "Settings > Home directory" write "/" (without the quotes)
explore the "efs" folder in the root
- move the following files in a backup folder somewhere to your internal SD card (you will need them if you want to restore later to original phone setup) : .nv_core.bak, .nv_core.bak.md5, .nv_data.bak, .nv_data.bak.md5, nv_data.bin, nv_data.bin.md5. Moving them they will be deleted from the efs folder (that's ok).
Now you will have it unlocked but with the generic IMEI.