There are two common causes for this error: You're eithe rusing a corrupted material or mental ray is flushing required scene elements from the database because it's running out of memory.
To exclude cause #1, simply assign a basic Lambert to all objects and see if the error goes away. If it doesn't, proceed to troubleshooting memory errors. I'd need more information than you provided to help you do that. The log file will tell you how much RAM was used, and the first thing I'd recommend is to check is the memory option in the mental ray Render Option window. Is Auto Memory Limit on for example?
Some common solutions to memory issues are: enable the 3GB switch under 32 bit windows, disable raytracing if you do not need it, break very large objects (especially those that surround the scene) into smaller chunks, replace omnidirectional light with a spotlight.
Thanks for the help! It was a memory problem... kinda. I made the mistake of not clearing out the history on all my objects. From what I gather, alot of history on alot of objects eats up alot of memory?? Did I hear correctly?