Layover in Milan

I say go for it! There's a lot to see within a few blocks of the Duomo, much of which isn't at all time-consuming: the Galleria Vittorio Emanuele, La Scala and its museum (you don't have to be a big opera buff to enjoy it), Sforza Castle, designer boutiques (which I couldn't afford, but they have amazing window displays) and of course, great people watching!

Tara,

You could certainly take the Malpensa Express from the airport and do some touring in Milan. Note that it terminates at Cadorna station and not Milano Centrale. However, that's a good area to go, as you can walk from there to Via Dante in about 10 minutes (a bit longer perhaps if you don't know the way - you could also take a Cab to the Duomo which is probably what I'd do under the circumstances). There will likely be Taxis at the station when the train arrives.

I have no information on luggage storage at MXP (not sure if you'll need that?).

One other point to note is that you'll probably be arriving at MXP Terminal 1 (which is where the Malpensa Express station is). Your flight to Sicily may be departing from Terminal 2, so you'll have to allow some extra time to take the shuttle between terminals (and collect your luggage if necessary). I was there a few weeks ago, but didn't need to use the shuttle so didn't bother finding out the details on the it.

Happy travels!

The last time we flew into Malpensa, it took, (and I am not kidding because I timed it) fifteen minutes from the time we stepped off the plane, cleared customs, and boarded the train to go up to Lake Como. The same train goes into Milan.

It might not be typical, but if anywhere close, you should have plenty of time to go into Milan and see what you like.