Road wheel conversion to single speed wheel
The Steamroller has 120mm spacing (i.e. track hub spacing) in the back (based on the 2011 Surly complete catalog). A modern road hub is 130 mm spacing. 

My recommendation is to buy a wheel with a 120 mm track hub on it and thread the cog on it -- you could spread (i.e. cold set) the frame in principle (spreading can be done if and only if the frame is steel) to take the 130 mm hub, but I would not recommend it (esp. on a nice Surly frame). Since it is more than a 1 size jump (120->126->130), you pretty much have to do the cold setting if you want to go through with this plan (you may want to get a bike shop to do it, esp. for the fork). 

But, if you do choose to take the plunge and spread the frame, the option you should probably take is get a bunch of spacers and a single speed cog and put the spacers on to get the right chain line. Many companies sell "single speed conversion kits", which sell the spacers as well as the cog together for a n-speed cassette hub, which you can get for a fairly low price. Less ideal options include things like just sticking the wheel in as-is (with a multi-speed cassette) and aligning the chain as best as possible, but if you're willing to buy an Eno freewheel, this is silly.