Yours truly has been investigating and I just can't think of a reason not to complete a case study if
A) Everything has gone swimmingly for a client
B) Enough time has elapsed to prove (A) is indeed the case
C) Nothing being implemented is of a particularly sensitive nature
We've done quite a few but it amazes me how (certainly initially) few people are keen to do Case Studies with business partners and/or vendors. This position I've always found surprising as it is a great was to showcase the fact everyone did a good job -- client end user/ IT within client / business partner / vendor --- and why would any of us want to shy away from that?
Oh - and this made me smile.