All of the above.
If you want to be a well known themer - build you own theme, host on drupal.org, tweet about it, offer help to others, support your issue queue, etc. If it's a nice theme, you should get a decent spotlight from that.
Outside of that, if you want to be a well known developer, you might jump on in with an existing module rather than build your own. There's thousands of modules out there all doing just about every task imaginable. It would take some imagination to come up with your own.
If you are a good developer, and are willing to learn drupal, a great idea would be to get in on existing projects. Most module developers would welcome help from others to keep things moving along. Big projects like Views, Context, Features, etc, would love to have you combing through their issue queues.
I've seen these scenarios several times before: guy helps with issue queue, guy gets named co-maintainer + gets mention on company blog, guy gets job there or with another company.
Issue queues are the perfect place to show your wares. best of luck!
Terry