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.
The main addition in tonight's build, as foreshadowed by the announcement title, is the ability to set the maximum number of active uploads past the heretofore limited number of just one. This started off looking like a relatively simple change, but ended up complicating things quite a lot, especially but not exclusively when it came to maintaining a maximum upload speed for several simultaneous uploads. I haven't been able to generate a very heavy upload stream on my end, so the whole thing is only moderately tested. If anything is not working as it should as a result, let me know and I'll issue a fix as soon as I can. As for the few extras; One among you Mac users complained that the default behavior of expanding newly browsed shares was causing a drastic slowdown on their machine. I could not reproduce this slowdown in my virtual OSX machine, but added the option not to expand new shares by default in a new UI section under the Options tab. Also in the new UI section, a Windows-specific setting that lets you decide whether you want the client to minimize the the tray, or to the taskbar as before. Unchecking this box will also remove the tray icon completely, since it serves no other purpose at this point. Finally, additional instances of SoulseekQt should no longer open if one is already running, which among other things should help when the program is pinned to the Windows 7 taskbar.
Nightly build 9/26 availalable on the download page as soon as I'm done updating the links.