Basically I think the BD setup with rigid fork is a perfect idea (and was exactly what I wanted). There is no need for suspension at the BD at least for my way of riding (touring on- and offroad, every day riding). However, I am experiencing two issues that I am a bit picky about.

1. Handling of my BD when riding turns: I have no idea whether this is a matter of tires, steering angles or fork specification. Surly setup looks perfectly alright. I cannot see any technical reason for this in frame geometry. Nevertheless, when riding fast turns with quite some slope, there is some sort of oversteering. Handle bar (or tire?) tends to take the turn harder and harder forcefully, no matter whether applying brakes or not. My BD does not just run through the bends in a neutral way as one would expect (my MTB or road bikes handling keeps neutral when riding turns with slope). I have not yet experienced such behaviour with previous bikes, regardless of tire thickness, but I have never been using Fat Frank tires with their special shape before. I have got an 18″ frame using a dainty hope headset keeping front low.
2. Combining rigid fork with disk brake: Apart from quick release axle issue, (see disk brake page) there is some more thing I would like to point out. When using brakes under certain circumstances fork might start building up oscillation in longitudinal direction somehow. Something I have never realized with disk brakes up to now, even at some previous rigid fork setup in another bicycle. It happens when braking on ripples or when using brakes in turns and trying to modify the curve radius. These actions will be taken up by fork which will start swinging back and forth (at least it feels as like), and brake lever needs to be released to calm it down. It is not just like using brakes on uneven ground, as like everyone is used to with ordinary MTB riding or so. In my opinion this may be a matter of rigid fork not being able to vertically absorb initial strong brake impulses on the one hand and riding impacts on the other (concurrently), making it start to oscillate. Maybe this is a matter of thick tire springiness lacking any dampening. Within the last 2600km of BD riding I had it twice, both when riding downhill tarmac roads with nearly no load. Second experience happened today in the morning when I took a tight turn with brakes applied and had to go even tighter which let fork start shaking.
   Brakes (2008 XT hydr. disk) provide strong, clear and smooth operation. There are no noises at all, no vibrations, nothing. Also, there are no issues with irregular brake power, bad rotors or anything.

I am currently thinking about testing my pace RC40F suspension fork (130mm of travel) at the BD to address both issues. It might absorb or dampen sharp braking and riding impacts and could probably prevent what I called fork oscillation. Also it will considerably change steering geometry and thus could resolve handling issue. However, these issues may very much be related to the tires used, so I should probably start experimenting with different tires as a first step.