AdWords could work if you can cheaply own the exact niche. The blog will take quite a while to draw any significant amount of traffic.

The best bang for the buck might be to get noticed by some high-profile blogger. Make a funny demo video like Dropbox and in-jokes for the Reddit crowd. Can your app do something like make Guy Kawasaki's or Scoble's face morph into a LOLCAT? Find a funny / interesting use and target some tech celebs like that. Or tell the story of how reading The Four Hour Work Week made you a success and Tim might pick up your blog post and link to it. Those are all somewhat long shots but I've seen them work and draw a burst of attention.