[troels]

As a designer, you usually have lots of sketches and early version lying around, as well as high resolution versions of the logo. That alone should work as proof that you are the creator.

Otherwise I guess you could e-mail a copy of your logo to your self. If it's a big provider (such as g-mail), the timestamps would be fairly reliable in a court room. Flickr is probably just as good.

[adldesigner]

As a designer, I can copy sketches or make high resolution versions of any logo and claim them as my own pretty easily. That would not work as proof that I'm the creator.

I believe metadata on the files can help. -I'm no lawyer though- or emailing a copy of the logo to yourself like you said.