Hi stenyak!
First, thanks for the bug reports! The one about the contacts has already been fixed and will be out on next release. The other one, you are right, it should warn you or directly not allow you to edit it when someone is editing. Now there's the possibility to checkout the document so that no one else can edit it while you have it checked out. Maybe that should be done automatically always when editing the document. We'll think about it.
We do not plan to support concurrent editing in the short term. The editor we are using now is an open source text editor called FCKEditor, and it doesn't support concurrent editing. We would need to work on our own specific editor or alter FCK and we don't have the time availability to do that yet.
However, we plan to support spreadsheets soon, maybe in a couple of months, and since that component is being developed by some members of the OpenGoo team, we plan to support concurrent edition for spreadsheets.
Cheers