I've made a fresh "test" install of WP 3.0.3 and added the Custom Login 0.8.4 plugin. Its bare-bones functionality appears to work, or at least the WordPress logo is gone from the login screen. However, I can't make it use my own logo.
I select the logo "upload" option, and pick the png file from my local disk. It sends the png to the appropriate content/uploads area. I notice that nothing is added or modified to the main settings form textbox for the logo path, even though it appears to have worked. Either way, I save the settings. When I demo the login screen, there is no logo.
My tests are on a Windows 7 x64 host. Perhaps this plugin is not compatible with a Windows host?