Hi,
Of the fanless series, I only worked with the DGS-1216T. Our older ver.B models are with fans (quite noisy), and the new ver.D units come without fans.
Quite a lot of things changed between revisions, judging from the outside (the boxes are under warranty and in production, so i can't open them). I'd guess DLink uses some better-and-improved circuits in the new models, which require less cooling.
My guess would be that if you run your units in a temperature controlled environment, and maybe put a few heatsinks on the chips, you can disconnect the fans on older units. Of course doing so would void your warranty, and is generally not considered a good idea.
Cheers,
Tamas