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.
If you've been keeping up with the nightly builds, the only major change since the last one is a reworking of the peer connection mechanism that better packs together the whole simultaneous direct/indirect connection business in a way that's a lot less likely to default on you as you exchange peer messages and/or transfer files with other clients. A full changelog of everything new since Public Build 3 is available further below. And, oh yes, Linux! Many caveats with that one. First of all, it's 32-bit. I haven't had much luck getting it to work in a 64-bit Linux virtual machine. And unfortunately getting a 64-bit version to compile appears to be a little less simple than running the codebase through a 64-bit toolchain, so for now it'll have to wait. You'll need the latest Qt libraries installed. I tried with just the Debian Qt4 packages and it wasn't enough. The easiest, most sure-fire way of getting the right ones is via the Qt SDK online installer (link below). You can choose a custom installation and de-select everything except Qt Creator (which can't be de-selected). The executable, also linked to below, can be run from anywhere once you've set the right permissions (chmod u+x). You'll notice some of the labels and buttons have their text cut short, and some of the columns not quite accommodating their content without resizing.. The default Qt font is bigger in Linux than it is in Windows, which will be addressed in future releases. I've also noticed upload speed limiting doesn't work anywhere as accurately in Linux. Not sure if it's something to do with the fact I was testing it in a virtual machine. Further investigation is requisite.
Tomorrow I'll be looking into the Mac version!