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.
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.
I can imagine having a URL like http://[known and trusted e-notary].com/[user]/[date]/[file or md5sum] would do a lot to discourage any shennanigans.
It wouldn't serve the same purpose as having something actually notarized so you'd have to be clear about that, but it could be a source of evidence when a case went before a court.
Doesn't seem all that technically difficult - md5sums for files and an RSA keypair per day are the common starting points for most of my ideas in that direction.
http://www.mozilla.org/foundation/trademarks/policy.html
Furthermore, the implication that words are trademarks and logos are non-trademarkable works of art is misleading. Names and phrases registered as trademarks are called word marks, and graphical logos are considered design marks. Both are trademarks (or servicemarks if you're offering a service).
This is a pretty useless discussion in any event. If you're wondering how to protect your logo, you should really ask a lawyer. All I'm saying is that your lawyer will tell you that you should trademark your logo, and that copyrighting it would be a complete waste of your time.
"Several categories of material are generally not eligible for federal copyright protection. These include among others: ... Titles, names, short phrases, and slogans; familiar symbols or designs; mere variations of typographic ornamentation, lettering, or coloring; mere listings of ingredients or contents."
Every legally protected logo I've ever seen is trademarked either at a state or federal level.
* Parent companies licensing their trademarks to subsidiaries.
* Unrelated companies using similar names who enter into an arrangement where one owns the trademark and the other licenses it. E.g. Tyco International/Tyco Toys, ITT Corp/ITT Tech, Apple Inc/Apple Corps (after Feb 2007).
* Certification, compatibility, or similar logos can be trademarked and licensed to authorized users so the owner can protect against the logo being used in an unauthorized or misleading fashion. E.g. UL Listed logo, Orthodox Union kosher symbol, Windows compatibility stickers.
* Franchising and similar situations. E.g. car dealers, fast-food restaurants, co-ops like IGA and Ace Hardware.
* Avoiding legal hassles that might ensue if the name/logo were not trademarked. E.g. Linux.
One obvious answer is "The US Copyright Office":
http://www.copyright.gov/eco/index.html
But that's $35 per registration. Seems like one of those online entrepreneurs I keep hearing about might be able to offer a cheaper service, via the web, to every designer in the world.
Of course, a lawyer may tell me that this is useless. Or it might be that the correct answer is "Flickr", and to do more is overkill.