MIDP 3.0 is the enhanced version of MIDP 2.0

The main enhancements are

Enable and specify proper behavior for MIDlets on each of CLDC, CDC, and OSGi, including :

• Enable multiple concurrent MIDlets
• Specify proper firewalling, runtime behaviors, and lifecycle management issues for MIDlets
• Enable background MIDlets (faceless MIDlets with no UI)
• Enable auto-launched MIDlets (started at device boot time)
• Enable inter-MIDlet communications

1. Enable shared libraries for MIDlets
2. Tighten spec in all areas to improve cross-device interoperability
3. Increase functionality in all areas, including :

• Improve UI expressability and extensibility
• Better support for devices with larger displays
• Enable MIDlets to draw to secondary display(s)
• Enable richer and higher performance games
• Secure RMS stores
• Removable/remote RMS stores
• IPv6
• Multiple network interfaces per device

1. Specify standard ways for doing MIDlet provisioning through other means (e.g. OMA (SyncML) DM/DS, Bluetooth, removable media, MMS, JSR 232, etc.)
2. Extensive device capabilities query
3. Localization & Internationalization (if appropriate, integrating/augmenting JSR 238 as needed)

The main thing is that it will be an open source & also backwardly compatible to MIDP 2.0 applications.

JCP specification:-  http://www.jcp.org/en/jsr/detail?id=271

source: Forum Nokia