Many of Milan's free city-sponsored WiFi sites already are activated (especially in the very heart of downtown), many more are on the way...easily findable with the online map ("Aree Servite") of the site...sigh...only in Italian (look for the green "pushpins"): OpenWiFiMilano....More......
Once in a spot covered by the free WiFi, connect with your 3-G+ cell phone, input your cell phone number as requested (it will be your USERID, according to the web site...I haven't tried it, yet!), and you'll be sent your permanent password in an SMS, active unless you don't use it for 365 days. If you don't have a 3-G+ cell phone, you can go to the ATM main center in the Duomo metro. Take care not to lose your PW!
Once you're registered, every day you'll have up to 300 MB of downloaded info (for those not tech-savvy, data is downloaded when you navigate, not just when you actively download pages/files). The first 60 minutes are at high speed, after which the traffic will run at 192 KB.