We are using Mac OS 10.7.5 and the uploads are working just fine across the system.
Are there any errors in the Event log? Have you tried using the development tools to inspect what are the responses and so on?
Would it be possible to provide us with access to the site so we can test it on our end as well? (you can send the credentials to email@example.com).
Are there any plugins or addons installed in the browsers? Which version of Silverlight is installed in them?