Ok it seems that this is "normal" (yea right!) for this phone.
This is from the owners manual:
From the cu400 owners manual Q&A page 82:
Q "Why does the phone heat up?"
A "The phone may get hot when there is a very long call duration or when games are played or even when surfing the internet for a long time. This has no effect upon the life of the product or performance."
You can try contacting the company you purchased it from (AT&T?) and see if they will replace it. As it is already a year old I somehow doubt that they will but it is worth a try.
I am sure that this is not what you wanted to hear, but it is the only honest answer there is. I, unfortunatly, have no control of some answers that I provide.
I would appreciate it if you would honor the time I have put into researching, and answering your question by clicking on the accept button.
Best regards; The Home Smithy
Once you have clicked on the accept button, please be sure to leave positive feed back for me.