Looking at http://net2apps.com/Product.aspx, they obviously were directly copying some of your graphics/design at one point. It looks like they've moved away somewhat.

You probably have a case, but I'm skeptical you'd recoup the cost of prosecuting it (in legal fees and in distracting yourself from executing.) Best case scenario is that they decide it's too much of a hassle and fold up shop; if they decide to fight they could change the infringing aspects and still compete with the same basic idea.

I doubt any company that stoops to this sort of thing is capable of out-executing you, so if I were you I would focus on doing your thing and let it sort itself out.