Here is a short thread on avs forum:

http://www.avsforum.com/avs-vb/showthread.php?t=873896

Quote from thread concerning phantom center:
"I will only caution that most players and AVR's will not allow this for Dolby TrueHD...."

I'm having a hard time finding definitive answers.

Another disturbing thing I have discovered in researching this is that most dolby digital tracks enable Dynamic Range Compression (DRC) in a 4.1 system. I'm thinking that is why I have always vastly preferred DTS over Dolby in my system.

If these limitations are true, it is incredibly frustrating. There are many situations where a phantom center can be a better choice. Placement of a center is frequently difficult and if the main speakers are of high quality (with great amplification) it can be expensive to match a center to this.