When you say it's "mostly flat" -- is there a way to put an approximate number on how many feet you're climbing every day?

I commute just over 10 miles round trip per day, with a moderate hill in one direction, about 340 feet of climbing. I usually do it on my fixed gear at 46x16t, and love it. It's easy to cruise fairly fast on my way to work in the morning, and easy to work myself out just a bit on the short climb back home.

However, I would not want to limit myself to only having a single speed bike -- or force myself to do longer climbs than this on it every day. If it works for your commute and some of your weekend rides, that's awesome. But it's not as fun to take out on group rides or really long rides, if you like climbing steeper and longer hills; it's possible on a single speed sure, but really not fun or reasonable. I consider myself a reasonably strong rider and can tackle some of the really steep blocks in SF fixed, but it's very taxing. I would not be able to climb longer routes on it, without a ton more training.

The advice of trying your commute in a single ratio on your current bike, such as 48x17, is great. If you can do that a couple times and don't mind it, go for a single speed bike for sure. And either keep around, or replace the mountain bike later, with another fun weekend bike for hillier or longer rides.