I've had very good experiences with my Surly Steamroller frameset. It has geometry similar to my Pacer road bike, which I find to be very comfortable and forgiving over long distances, yet still aggressive enough to go hard and fast. While I have only had it in a skinny tire, track bar, go-fast setup in the time that I've had it, I'm excited about exploiting the versatility of this bike to ride on more varied/off-road terrain. A swap of chainrings, a change of bars, and a variety of tires should accomplish this purpose. A perusal of FGG will reveal numerous setup options (I particularly like Todd Burpee's: http://www.fixedgeargallery.com/2006/mar/NT650_aol.htm)

Here's where I'd ultimately like the bike to be:

The platform:
62cm Steamroller frame
Double-fixed high flange rear hub, Velocity Aero rim
High flange front hub, Velocity Aero rim
144bcd Suntour Superbe cranks
17t and 19t rear cogs
42t, 45t, 47t, 49t, 52t chainrings
Front brake and 'cross lever

Go-fast:
23c/25c tires
Track bars
Negative rise stem
49x17 (76") or 52x17 (80")

Off-road:
35c/38c tires (whatever fits)
Mustache bars
Positive rise stem
42x19 (60")

Commuting/errands:
23c/25c tires
Mustache bars
No rise stem
47x17 (73") or 45x17 (70")

This is definitely a good bike for obsessive tinkerers like me