Characters that you cannot use in sites, folders, and files in SharePoint Portal Server 2003 or in SharePoint Server 2007

• tilde (~)
• number sign (#)
• percent (%)
• ampersand (&)
• asterisk (*)
• braces ({ })
• backslash (\)
• colon (:)
• angle brackets (< >)
• question mark (?)
• slash (/)
• plus sign (+)
• pipe (|)
• quotation mark (“)

Source: http://support.microsoft.com/kb/905231