1. Is is possible to connect with an amiga to a linux system using openssh?
Yes. I use
AmTelnet. There are also free(ware) ssh ports for amiga, try google. These don't do ssh2 though, so you need to tune your sshd_config to have "Protocol 2,1".
2.Is it posible to use som implementation of xwindows and above-mentioned openssh connection to get apps running on the linux system to appear in nice little windows in the amiga workbench?
Try
xami.
However, I am not sure if it does ssh tunneling at all. I use it in fullscreen mode.
Don't forget to enable networking mode in your linux box xserver (for example Debian GNU/Linux has it disabled by default, for security reasons).
3. If the answer is yes to any of the questions, is there any limitations to the amiga version of these programs...as in: Well, it exists but haven't been updated since xxxx, and only works with old versions of x or ssh or whatever?
AmTelnet only does ssh1. Not sure about those "free" ports.
Xami is quite recent, and it works really great in LAN.