[gojomo]

(You mean "prove original authorship" rather than "prove prior art".)

The act of publishing might create a reliable record of first authorship, for example if the material becomes quoted or replicated in third-party systems with timestamps not trivially changeable by the disputants. (Simply using a web host or blog service not completely under the writer's control might create such records.)

But, if you really want to prove authorship as of a certain date, you may want a timestamping service:

http://en.wikipedia.org/wiki/Trusted_timestamping