As a B2B marketer I'm watching the Twitter phenomenon closely and with great interest. I'm not so sure people are interested in following brands. I may follow a brand if it offers me something of value in the way of advice, service and ideas. I won't follow it to become another "lead" on the web 2.0 version of an email blast list.
Twitter is kind of like the "Long Tail" for creating your social network of contacts. These are people who choose to follow you for various reasons and motives. Some are trying to build their lists and sell you on how to make $10,000 per month using Twitter. (I block these). However, many people are using Twitter to share content, ideas and connections. These people I tend to follow back.
Using Gmail to track followers
To make things easy, I have set up a filter rule in Gmail, that puts all my follower notices in one place (see screenshot on the left). I can see how many people have followed me based on date. It also confirms which Twitter Followers profiles I've check out and who I haven't. (read emails means I have, unread means I haven't).
I'm not actively building a Twitter list, it's organically growing and I don't automagically follow someone because they follow me. I do get to know them a bit though. I check their profile, their tweets and their blog or site. If within 30 seconds or so, I see something that tweeks my interest I'll follow.
You'd be surprised at who is following you. It could be your next customer, your current customer or a prospective partner!