ATL was the world's busiest airport in 2007. If the weather isn't an issue, a 1 hour connection is no problem. You didn't specify when you are planning on flying but ATL is prone to thunderstorms in the summer.

If you can't fly in the night before the wedding, I'd recommend taking a nonstop flight from MKE to DEN on another airline rather than a connecting flight. This should reduce the chance of any problems. Frontier and Midwest aren't in the best financial position, so you may want to take that into consideration if booking a flight a number of months in advance.

You can use FlightStats.com to get more information on the on-time percentages for each flight.