I have a single speed cassette hub on my single speed mountain bike (a Hope Pro 2 trials hub). The stubby cassette gives you enough space to dial in chainline and also gives you wide flange spacing for a strong, dishless rear wheel. Makes swapping gears depending on my riding location really simple.

While the profile hub is wicked expensive, it's a nice solution for people w/ 120mm spaced frames to have the same ease of cog swaps as mtb hubs have had for ages now.

If I had a really nice single speed 'cross bike w/ 120mm spacing, I'd be tempted to spring for the Profile setup. Come to the races w/ a couple of different cogs ready, so after your course preride, you can easily swap out cogs for your race if the conditions call for it. Right now for 'cross races, I bring a bag of White Industries freewheels (at around 80 bucks a pop, sizes 16 thru 19) and sometimes have to really muscle the existing freewheel off, depending how long it's been on my wheel. Even with sufficient grease, freewheels can sometimes be a huge PITA to remove.