Personally, I’m not particularly keen on the idea of using Twitter for affiliate marketing beyond promoting your site directly. It just reeks a little too much of spamming a social network.
It’s a reasonable argument that there are a lot of people who are currently using Twitter as purely a promotional tool for pushing their online products or services to a wider social market however affiliate marketing on Twitter is using a social tool to trick a user into clicking on a commissioned URL. That’s pretty close to spam in my books.
A big part of Twitter is the use of short URL services and there is no way for a user to know that they’re clicking through an affiliate link. If you’re going to be completely up-front about the use of affiliate links in shortened URLs then you should be notifying the potential clicker that the link is an advertisement or at least that it’s an affiliate link.
There’s nothing wrong with using Twitter to promote your affiliate site (particularly if you have good content on it), that’s what a large portion of people on Twitter are doing however even in that scenario it should be relevant content that you’re posting on your site for your Twitter followers.