Completely agree with Jorge's comments (and I would have upvoted it and commented there rather than here if I had enough points!)

I posted over on http://answers.onstartups.com/, and then was redirected here, asking a very similiar question, and I received some good answers (http://answers.onstartups.com/questions/1994/startup-idea-and-web-domain-is-it-being-done-already)

I think the COM domain is always much more memorable.

The average internet user may remember a site name, but does not often really understand the distinction between the various TLD's (COM, NET, ORG, EDU, etc). Plus, if the browser often fills in .COM automatically, if the user doesn't specify.

So, you hear about folks who buy own the COM version of a popular NET domain, and so they receive the email (inadvertently) and all the extra traffic -- this is traffic that should be going to the .NET, but ends up at the .COM.

One the other hand, if you have (or plan to have) a famous branded COM domain, then buying the .NET and .ORG versions of the domain can be a good defense against cyber-squatters.