I hope not. I like SourceTree the way it is (on OS X) and every cross platform UI toolkit I know produces apps that don't feel native.

IMHO Bitbucket/Atlassian is doing it the right way: Native on the client side.

Check out Xamarin Studio. It's a full blown IDE and it's quite impressive in terms of native feel. Apparently written in C#, Mono and GTK#.

http://xamarin.com/studio

They have stayed pretty true to it, they bought it about a year ago and made it free. It used to be like Tower or Cornerstone where you had to buy it for $29 bucks or something. Have liked it since then and was a much better client than MacHg at the time for teams. Glad Windows is getting this, I use TortoiseHg and Git there currently but will probably switch.

They would have to rewrite their UI, because Mono does not support WPF. In fact, they could be using a whole plethora of features not included in Mono. Mono is a wonderful project, and I've found it very useful, but the .Net -> Mono transition is still a huge time sink.

SourceTree for Windows appears to be written in C#, but their UI is built with WPF which I believe will be a limiting factor for cross-platform compatibility.