According to this interview with Jim, he confirms the origin of "the bet".
How did you come up with the original idea for CODEX ALERA? We’ve heard rumours that it involved a bet on whether you could combine the Roman empire and Pokémon… is that true?
The bet was actually centered around writing craft discussions being held on the then-new Del Rey Online Writers’ Workshop, I believe. The issue at hand was central story concepts. One side of the argument claimed that a good enough central premise would make a great book, even if you were a lousy writer. The other side contended that the central concept was far less important than the execution of the story, and that the most overused central concept in the world could have life breathed into by a skilled writer.
It raged back and forth in an ALL CAPITAL LETTERS FLAMEWAR between a bunch of unpublished writers, and finally some guy dared me to put my money where my mouth was, by letting him give me a cheesy central story concept, which I would then use in an original novel.
Me being an arrogant kid, I wrote him back saying, “Why don’t you give me TWO terrible ideas for a story, and I’ll use them BOTH.”
The core ideas he gave me were Lost Roman Legion and Pokémon… Thus was Alera formed.
Jim has responded similarly all along, so the story has been out there a while and occasionally warped a bit. I can't find any direct link to the debate, it may not have been publicly available online at all (not familiar with the forums he quotes).
Here's a Youtube link with a SDCC Q&A where he goes into the story as well. He elaborates on how he delved into the two "bad ideas" to get the good stuff (which I had no idea of, so thank you for prompting me to find this) He also claims here that he went back to the debate and claimed he "lost", so that he could properly publish the resulting awesome.