I came across an OMG!Ubuntu! article that may improve things. They say
Using ‘proprietary/restricted’ drivers for Nvidia and ATI graphics cards often results in a less-than-perfect looking boot screen.
Although purely a cosmetic flaw – it doesn’t impeded the boot speed or performance – its one that greets you time and time again. Thankfully the script below does, in most cases, ‘fix’ it.
If this has been an issue for you, check out the article.