Becoming a professional Twitter for a company could be your first route, most companies aren't prepared for the social media realm but are longing to join, they need people who are already expert and all you'll need to do is look on the job boards for "Seeking Twitter Pros".
You could sell stuff on Twitter, anything at all, "Special offers for Twitter followers" are now commonplace, try 'Micro fundraising' by combining micro blogging and micro funding.
You could try your hand at promoting music. A service called Tra.kz combines a short URL service with a music community and a marketplace, 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, neither seem spammy but do appear professional enough although as yet I don't know if you can earn proper money without your followers being scared off.
There is another Twitter ad solution and that is Twitadd, this places banner ads on your Twitter page, why not try a little affiliate link, not always welcome but there's no harm in receiving commission from something you already support
There are business opportunities to find on Twitter, there are daily Tweets saying stuff like "Looking for x programmer", many companies offer Twitter Apps for free, there are more and more professional applications appearing on the web, applications people would pay for.
You can't really review a product on Twitter but you can certainly mention it, but it does appear unfair that you can't advertise your content.
Lastly, it may be simple and limited but Twitter's uses may be many and varied, soon there may be companies who will require Twitter consultants and specialists to teach them how to use it in their particular niche market
