Finally I found some time to deal with most of them, something that has kept me busy for around three weeks (and which I should have done much, much earlier). All the issues that were resolved are detailed in this other post.
There still are some problems in the code (which code doesn't have any?) but these do not prevent tmpfs from working fine. Of course they should be addressed in the future but people is already enjoying tmpfs in their installations and have been requesting its activation by default for a long time.
Hence, after core@'s blessing, I'm proud to announce that tmpfs has been marked non-experimental and is now enabled by default in the GENERIC kernels of amd64, i386, macppc and sparc64. Other platforms will probably follow soon.
The next logical step is to replace mfs with tmpfs wherever the former is used (e.g. in sysinst) but more testing is required before this happens. And this is what 4.0_BETA will allow users to do :-) Enjoy!