It took me a while but I think I managed to get mine working in 10.7 Server on a Mac mini.
I turned on web sharing in system preferences then set up file sharing in the server app
and assigned groups to the sharepoints.
Then I created user accounts and added them to the groups
and then I logged into those accounts on the server, to cement their account type.
After that, the accounts are accessible via https://<your-ip>/webdav/ from a Mac and from an iPad. But as somebody else pointed out, you must use the "short name" not hte full user name, to get in.
Windows access works but is a bit flaky, depending on what version of Windows. In W7 just right-click on the "computer" folder in an open window, and choose "add network location" then follow the instructions. But I had to specify a singke folder, such as https://<your-ip>/webdav/thisfolder/ to get it to work.
The slowest part for me was realizing that I needed to use an account that had already logged in on the server as a regular user. And then making sure that the user and group permissions and access lists didn't accidentally lock out a user.
Hope this helps!