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Usually they want the demographics on who your readers are (ie. men, women, age range, etc.) How many subscribers, RSS, newsletter that you have. Besides numbers I think that is a good start. I feel like I am missing something, if I think of it I will be back :)
You can get info by using Quantcast.com. If they don't have your blog listed (they probably won't unless you're getting very good traffic) you register an account and put code on your blog. It'll take awhile (at least a few weeks) to get the demographic info and the longer you're using Quantcast.com, the more accurate your demographic info will be.
Until then, you can guesstimate your readership info by who comments on your blog and what search terms they use to find you. (For example, moms and dads are probably searching for info on baby items.)
I just signed up for Quantcast but what i did when someone wanted readership info before I had any was a created a quick poll on my site askign which of the following applied: man, woman, parent, teacher, librarian, grandparent, teenager etc... (I used teacher and librarian because it was my book blog I did this on) anyways I asked all my reader to please vote and then i sent that data to the PR person.
For readership info, I would state the majority of my blog readers/followers- such as, if you have a mom blog, your readers might be your fellow mom bloggers ages 20-60. Also state what social networking sites you belong to like Twitter, Facebook, Entrecard, etcetera. You can also state that on the question 'how you promote your blog".
I would go back to your client and ask them to clarify "readership info" for you. You don't have to make it seem as though you don't know what they're asking for. By asking for clarification you'll be able to provide them with exactly what it is they're looking for and none of the other gobbledygook.
You could also try using the utility at www.compete.com. You can sign up for a free membership and compare your site with up to five other sites. Then you can pass along that comparison (if it makes your sight look good) to your client.