Just released GMime 2.3.2 last night which took some comments about the GMimeHeaderIter API into consideration (like allowing iters to be on the stack).
It also now has support for multipart/encrypted PGP/MIME parts that have been both signed and encrypted in a single pass (support for this exists for both encrypting and decrypting).
Many more .NET API improvements as well.
Check it out!
Updated: Just released 2.3.3 after a weekend of hacking.