Close button sends client to the tray, statically linked Qt for OSX.
See the Changelog for list of features and fixes currently only available in the nightly builds.
As I've had to port SoulseekQt to 64-bit Mac, I do intend to start offering 64-bit versions of the SoulseekQt client for Linux eventually. Until then, user frd was kind enough to post instructions for getting 32-bit SoulseekQt running on 64-bit Linux. You can read them here.
Update: User Dewb wrote an Ubuntu specific guide that also shows you how to integrate SoulseekQt with the Ubuntu shell.