Considering it's looking less and less likely that Windows Phone 7 will support Twitter directly (though see here for contra), Seesmic's app looks to be a winner.
Like all Seesmic Twitter apps (e.g. desktop, Android), we can expect this to be a free offering as well. Can't be that.
Best part in our opinion? The ability to customize the look/layout of the program, followed by the solid and evidently seamless integration with the OS.