IT organizations striving to improve their business should or must be looking for various models and frameworks beyond CMMI. ITIL is the obvious next step. While CMMI is focused towards software development,maintenance, product inegration etc., ITIL provides a framework specifically developed for IT Service Management and operations. IT Services Management is being increasingly adopted by organizations to ensure that the IT processes are closely aligned to business processes and that IT delivers the correct and appropriate business solutions whether it is for Corporate Business Change Plans or Outsourced Application Management or Outsourced Infrastructure Management or BPO.

Important question then to answer is:
How to get the best RoI from the efforts of implementing these models? Is there
any relationship or overlapping among these frameworks?

With our CMMI implementation, we also answered these questions.
There is definite scope of mapping between ITIL processes and various CMMI practices.
To give a heads-up, some of the key CMMI processes that maps to the ITIL processes are:
Project Planning
Project Monitoring and Control
Integrated Project Management
Measurement & Analysis
Configuration Management
Requirement Management/Development
Risk Management
Validation
Product Integration etc.

And they map to ITIL processes like:
Problem Management
Helpdesk
Incident Management
Change Management
Configuration Management
Service Level Management
Planning and Control
Contigency Planning
Security Management
Availability Management
Capacity Management etc.

Digite's Universal Process Framework makes it all the more easy to implement ITIL process templates. However, one caution: Amidst all the process improvement framework, one shouldnt get into "process obsession" mode. Assess the level of risk in the processes, the need for the extent of management control and regulatory requirements and focus on "just enough processes" -- facilitate the process improvement to make a significant difference in your IT services delivery or business performance, instead of overburdening yourself with process mania.