I believe this bug is carried over from ubuntu (bug 147623 and others). There is a black box, or blank box that blocks some of the usplash image for widescreen lcd users with newer nvidia graphics cards. Since the release of Gloria/Jaunty; the framebuffer detection and usplash resolution has been corrected, and if not; can be corrected by adding vga parameters to the boot kernel line in menu.lst. There seems to be no reported fix for this bug that works to remove the odd text box that appears with certain hardware, which also includes using the vesafb and fbcon framebuffer modules (This fix worked for me in previous releases, but has no affect in Gloria x64). As of now, I have removed the usplash theme and prefer to view the boot messages, instead of the messed up usplash image/animation. Some Jaunty amd64 users claim to have fixed the problem by reducing their usplash resolutions; however, I believe this only moves the blanked box area away from the center of the image, so that it's no longer visible against the same colored background.
Thank you for reporting a bug in Linux Mint. Unfortunately I do not have the necessary hardware to reproduce your problem. If anyone else can confirm this bug, please say so. Please check out the Linux Mint Forums, where there are many more people available to help out with diagnosing any problems you have. Please repost this in the forums, and put a link to it here, so I can track any further development. It is also exceptionally useful if you include the output of "inxi -F" when reporting hardware problems. This is a standard response, so it may not be entirely appropriate.