The original SourceTree for Mac is a native Cocoa app. If this Windows version has been written in C# it could potentially be ported to Linux with Mono and GTK#/Qyoto. Perhaps the future is one single cross-platform app?
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.