[Wiktionary][1] has it:

>1. *(transitive, Internet)* To blog something previously posted on another blog.

Basically, even if most people wouldn't recognize it, it's technically a [word][2] as long as someone uses it to communicate an idea.  Some people add to the definition to say that it must be widely used in order to be really recognized as a word, but in that case, the issue boils down to recognition, not the word's status as a "word".  I believe it to be used enough to be considered a "word", even by the latter definition.


  [1]: http://en.wiktionary.org/wiki/reblog
  [2]: http://dictionary.reference.com/browse/word