Becoming a professional Twitter for a company could be your first route, there's a lot of companies out there who long to join the social media realm but don't have the expertise to pull it off, so they'll be looking for someone with that knowledge and all you'll need to do is look on the job boards for "Seeking Twitter Pros".
You can sell your stuff on Twitter, even big companies do it, why not try 'Micro fundraising' by combining the two hugely successful "micro" ways of doing things, micro blogging and micro funding?
You can try promoting music by combining a short URL service with a marketplace and a music community, which sounds a lot better than the music industry or the iTunes monopoly.
Advertising is always a good way of earning money online and there are two Twitter ads services I've noticed so far: Magpie and The Branch, they both look professional enough not to appear spammy but as yet I haven't tried them so i don't know if you can earn real money without scaring off your followers.
There is also Twitadd, a Twitter ad solution that puts banner ads on your Twitter page, you could also sneak in affiliate links, not cheating as such but there can't be anything wrong with getting commission from services you already support.
Look for a freelance job on Twitter, there are Tweets daily saying "Looking for x programmer", offering Twitter Apps, many companies offer Twitter Apps for free, recently more professional applications appear on the web, applications people will pay for.
Although you can on a blog, reviewing a product on Twitter is at the moment frowned upon, but it does appear unfair that you can't advertise your content.
Twitter may be simple and limited but it can be used for so many different purposes, many companies will soon require Twitter consultants to guide them through the uses of it in their own particular niche market.
