gc_app2007,
Then you shall be all right. Search this forum, there are lot threads with valuable information on this topic. I searched with the key words "indefinite permanent position". It came out couple of threads. Following is an example:
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needs letter from chairman and /or dean stating that your iimediate boss can hire you as permanent employee...something like
This is to certify that NIW_2005 is a fulltime permanent employee at XXX at the University of XXX He is currently working as XXXat the Department of XXX. He joined this position on XXX and was offered to work on this position for an indefinite time. Along with the salary, XXX is awarded several benefits such as health insurance, retirement plans, and tuition reduction. I also wish to certify that Dr. XXX at the XXXX had full authority to hire XXX as a Permanent Research Employee in their research laboratory.
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The above quote is from
http://www.immigrationportal.com/showthread.php?t=209032
I don't think you can get any official offer letter stating the permanent position. Even for tenure track faculty (I am one), we still get the renewable offer letter. The key thing for your case is to get a statement from your chair or dean to confirm that you are there to work for an indefinite time.