castor jdo precedes java's jdo, and therefore is not an implementation of java jdo.
in my opinion, jdo is still a fairly new untested technology. i played around with the reference implementation a few months ago and was not wowed by it. i had a lot of trouble getting the code modification stuff (enhancement, i think its called) to work. i'm sure it was my goof, but it was not very informative in telling me what was wrong.
most useful (java) jdo implementations, afaik, are not free. i see that there *is* one open source implementation on sourceforge, though
tjdo