I’m no expert on WML, but it sounds like you have a problem with the Content-type headers generated by Apache.
To see what headers (and content) you’re sending out, you can use http://paulding.net/vsource.html
“Filename.xml” should get a header
“Filename.wbmp” should get a header
“Filename.wmls” should get a header
(There will surely be other headers as well…)
Apache should handle the first two OK with the default configuration, but the third is missing from the standard mime.types file. You can add it in your .htaccess file with the line
AddType text/vnd.wap.wmlscript wmls
Note that AddType not only allows you to define new types, but allows you to overrule the standard definitions as well, in case your the client software wants something other than what Apache is generating.
Hope this helps!