A bit different from a lot of the current builds being discussed, but hey this is the FGSS subforum not the track forum but I want to go with a SS/FG mountain bike (already have my fixed road bike). Assuming it doesn't throw off the geometry too much I may go with a 69'er (26" in back 29" in front).

Haro Escape A2 Frame (Its silver and has a beautiful curved top tube)
Replacing suspension fork with rigid straight blade Zion fork.
Replace mountain wheelset with cyclocross wheelset flip flop hub in rear.
Cyclocross knobbies
Keep current crank although drop the granny ring and throw on a bash guard if possible.

I haven't decided on the cockpit yet. Currently since it is setup as my aggressive mountain bike (I'm switching to full suspension so hence the want to rebuild this into a SS/FG) so it has wide AM/DH risers. Anyone have any suggestions on the bars or stem? Maybe go back to a flat bar? Shallower risers? I want to really be able to cruise fast on less technical terrain, fire roads, etc.