Clement Lefebvre, the Linux Mint founder, recently talked to Muktware about Banshee - which has replaced Rhythmbox in the recent Ubuntu 11.04 development builds -, and Mono. Banshee is an amazing application but it uses Mono which many consider risky because Microsoft could one day assert its patents against all the free applications that use Mono.
While Clement says he doesn't consider Mono a thread, it looks like Rhythmbox will stay as default in Linux Mint:
Rhythmbox is our default player and we're quite happy with it. If Ubuntu switches to Banshee, we'll of course give it a close look and assess whether the migration is a good move for us too, but as it is now there isn't any plan to change anything.
As you probably already know by now, Linux Mint will not switch to Unity either, keeping the old "classic" GNOME as default.