Hey Buddy,
To address your previous question... I would definitely not promote other non-php or non-website programming related stuff. This may make them think that they are getting spam and they would unsubscribe.
Lisa made some great points!
Adding another field would be great, but to get to know a little bit more on who is in your existing list... you can come up with a survey of questions about what you want to know about them, and email them a link to the survey. In your email subject:
a) you can appeal to their good nature by having your subject "I need your help!" Or b) you can appeal to their capitalistic nature by bribing them with a bonus.
Of course, the point is that they would be helping you to help them (i.e. to serve them better), so they may go for it.
Maybe this is tacky, is it? I don't know. I am just thinking out loud.