I have a Brooks Flyer that I've put about 600 miles into (about a month). At first the leather was incredibly rigid. I remember thinking that it felt just like sitting on hard plastic and had thoughts of returning it if it didn't improve. It has improved quite a bit by riding, and I think it will only continue to get better.

According to Sheldon Brown's site, you can use oil to help the break in process. Proofhide works also, but is much slower (again, according to SB's site).