I'm having the same issue on Win7x64 running IIS 7.5 (PHP/MySQL installed)...
I've triple checked the permissions on the file, and the folder itself - it's all writable...
Is there a PHP version requirement perchance? I'm running 5.3.13, but - I'm having no problems writing/updating files with anything else I'm working on.
I assume it's just Microsoft being Microsoft.
Edit: Disregard the PHP req. comment - just noticed minimum version required is 4.3