I have read a few posts, and also have seen a brand new out of the box Nokia E71 that will only read MicroSD cards when there is no SIM card in the phone. It could be that a few handsets have got through Nokia's Stringent quality control.
On the face of it, it seems that there is some kind of defect with the handset that maybe causing this to occour.

The picture above, shows how close the internals of the E71 are.
The only explaination I have been able to find is that the SIM Card which is to the rear of the memory card holder, and on the same horizontal plane as it, is putting undue pressure onto the memory card holder and this is probably causing the memory card contacts come out-of-contact with the MicroSD card and this is the cause of the malfunction.
The only course of action in this case is to take your handset to a Nokia Care Point where an assessment will be made of the handset, and if it is in warranty then a repair may be made.
If you are a prospective purchaser of the E71 - it may pay to check the phone at the store where you are buying it (if possible) to check that this defect does not exist, at that point the store should then provide you with a functional handset.
For information on finding a repair point, this helpful PoK article will direct you:
/discussions/board/message?board.id=topfaq&thread.id=3
Edit:
This PDF:
https://gullfoss2.fcc.gov/prod/oet/forms/blobs/retrieve.cgi?attachment_id=932229&native_or_pdf=pdf
Shows internal components pictures issued by Nokia to the FCC for type approval, (publicly exhibited by the United States FCC as part of the approval process) as we can see on page two and four of the PDF, showing the internal circuit board arrangement of the prototype the memory card is quite close to the SIM card container.
Shunts...
Message Edited by shuntfield on 10-Feb-2009 09:12 PM
I will mostly be communicating with a
Nokia E72 Zodium BlackNokia E72-1 with Vr 051.018.207.04 Software
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kudos!!