Couple of things I would look into:
1. Corbis allows comp downloads of images. Be sure to ask how those comps are used. I've found many of my images on other sites and am still trying to find out how they got there and if they were licensed.
2. Find out if your images will be available on any other licensing websites that may or may not be affiliated with Corbis.
I contribute to Retna, who is partnered with Corbis. I didn't realize that my images would also be listed on Corbis, but as long as I get paid for any licensing, then I have no problem. However, I have also found my photos on some Japanese site available for licensing, which I believe were obtained from a comp download through Corbis, although I'm not sure if this site has any affiliation with Corbis or Retna, so I can't say for sure but it was one of my questions I'm waiting to get answered. I spoke to someone at Retna two weeks ago inquiring about these concerns I have, and after waiting over a week still have not yet received an answer to my questions.
Also, they don't send me monthly statements, which they are supposed to do.
If you have any questions, I would call them and see how they treat you on the phone and if they take the time to answer your questions.