Several websites that I have researched say that a mezuzah parchment must be protected from dampness and at least should be have a plastic film on the outside, i.e. it can not be completely inserted into a plastic bag. (See this.)
Most places recommend placing the mezuzah into a case (made of ceramic, wood, etc.). I assume that the case makes it easier to secure and also provides extra protection.
The only place I can recall having seen a mezuzah on the wall without a case was in my friend's work place. My friend is Orthodox, and I assume that he knew this was OK. But then, again, I have also read opinions that since one's office is not a permanent residence, one is not required to have any mezuzah in is office. So, it's possible that he may have just had it there to be "decorative", and it wouldn't really matter halachically in his case (no pun of the word, here) if he used a case.
Halachically, is the case absolutely required? Can one roll the parchment, and just secure the parchment to the wall using tape or any other method that would make it secure?