I can confidently answer some of your questions:
Since 1.05b1 has been out for a month now, anyone seeing any major issues?
Based on my personal observations and postings I've read to date, I have not seen any critical issues. In fact, firmware v1.05b1, corrects at least one issue in the v1.04 public release.
Proper support for AF drives, I assume, will require a reformat. Anything I need to be aware of? I'm running 4 2TB WD Green drives in RAID5--anyone else in this configuration or similar notice any issues (or speedups?).
Personally, if I had a RAID 5 array that was running stable for a prolonged period, I would be hesitant to do anything drastic (like a complete reformat) that could possibly jeopardize array stability. My DNS-343 is configured with standard volumes rather than RAID, so I can not comment on the impact the AF support will have or any benefits accrued from updating.
For the D-Link engineers: any idea as to rough timeframe? I'm fine running beta firmware if it's stable, but I'd rather wait for final release (moving 2TB of data around is slow enough...) if it's going to happen in the next month or so.
The release date for the next firmware update is indeterminate. Based on personal experience in this forum, I would not count on a new firmware release in the coming month or two (although I hope I'm wrong).