As with all zope objects (unrelated to webdav), here's the non- 
exclusive list.

file names cannot contain non-ASCII characters, the valid characters  
are: "a-z", "A-Z" and "0-9". Characters containing accents are not  
valid.

file names cannot start with any of the following: "_", "aq_", ".",  
"..".

file names cannot end with two underscores: "__".

file names can contain the following characters: underscore "_" (see  
previous notes), tilde "~", comma, period, dollar sign, parenthesis,  
hash "#" and spaces " ".

file names cannot be the same as any file inside the Plone skins  
directory.