>Straight forward question and I would like the opinion of HN: Can you build a successful product/service based on Twitter?
Absolutely, look at Summize (now Twitter Search) and Friendfeed who has a large amount of data in the form of Tweets.
You also have the clients, eg Twhirl (purchased by Loic Le Meur) and Tweetdeck which is another format you could follow. I'm sure some iPhone apps that focus on Twitter do very well for themselves too.
There is still plenty of opportunity out there. I had an idea for something that wasn't a part of the API last year (I emailed Alex asking if it was due anytime and he said it was on the list). I just checked the API now and it seems like its there, so I may give it a go soon.
Honestly though, I'd say start with small ambitions on scalable infrastructure (eg AWS) and grow from there. You'll know if you hit a chord as stuff spreads through Twitter like wildfire.
In fact, given this trait of Twitter, I'd suggest its a perfect protocol to build/iterate upon as you get almost instant feedback on whether you're going the right way.