1) Does the installation remove Linux/Qtopia or is it an bootloader where you can choose?
At the OpenBSD bootloader prompt, if you type c[enter] you load linux, otherwise wait or hit [enter] and you load OpenBSD.
4) which applications are included besides POSIX and some X11 tools?
Base os includes a few things that might be packages on some other OS, e.g. sendmail, OpenSSH, httpd, named, netcat. Compilers are treated as an optional part of the OS, rather than as add-in packages. Some
precompiled packages are available. Most of the rest of the ports collection can be compiled yourself, some might not be compatible with arm though due to unportable coding.
5) is there a way to get rid of it if it does not work as expected?
If you decide to install it in the first place, I don't know why you'd want to remove it (-:
6) which version of gcc does it use (2.95.x, 3.x, 4.x)?
3.3.5 with local patches, see gcc-local(1) for info about these.
7) how fast is the floating point emulation?
Don't know - int-only decoders (libmad/tremor) avoid floating-point for listening to music. I can't think of other software I've tried that would make much use of FP.
8) does communication for Ethernet/IP over USB work the same way as for Linux? (would it break the AJZaurusUSB driver for MacOS X?)
USB is only supported as a 'host' not as a 'device'. You need a Zhost cable and either a nic, or some other host-to-host cable, if you want to connect by USB. Maybe CF wi-fi is an easier option.
There's also no support for the internal flash (so sl-c3100 doesn't give a benefit over sl-c3000), or for SD cards in the built-in reader (though that's listed as a project on
http://www.openbsd.org/zaurus.html).
It is a standard Unix installation, not the easy end-user PDA-style experience of Qtopia (or even a desktop-oriented Linux). The general experience is very similar to OpenBSD on i386 and the other architectures - that's the whole point. Expect the installer to leave you with a shell prompt on a tty, not a GUI login screen and preconfigured desktop.
If you want to see if it suits you before installing on your Z, try it out on a spare computer (PC, Sun, [...]) with a blank or unneeded hard-drive (or e.g. vmware player in the case of a PC).