I work for one of the "big banks" in the US and happen to have our mobile product offers as a subject matter expert area. These days, there is very little risk in banking via a public hotspot, especially if you're using the mobile app provided by your bank. The way the information is passed over the network the chances of something happening are slim. Now that said, using a mobile browser is also just as safe but it's easier to find a way into. Your bank should have a mobile security gaurantee in place that protects you as well.
NOTE: These opinions are my personal opinions and are not associated in any way with the bank I work for. (we've been asked to say that whenever we reference our employment, even if not by name, so I'm being a good little employee).