Ian Bicking comments on some frustrations he had in regards to using Python's pickles to store data.
His comments remind me of the conclusion I came to when I started to wonder why OODB's didn't catch on in a bigger way along with OO languages. It always seemed to me that an OODB was a better match to an OO language than an RDB, but still we do most everything in a relational database.
I believe the answer is the same as the conclusion that Ian came to. Transparency. It's more important to make the data portable than it is to make the code portable, and an OODB is too tightly coupled to the OO Language implementation (caveat -- this is true for the OODB's I've looked into, and that is hardly an exhaustive list). Ten years from now, no one will care what the code looks like, but they will care tremendously what the data looks like.