Hi Kate,
This question comes up periodically. Here's a brief summary:
Technically some journals will consider a poem published if it's been posted anywhere on the net, but in practice this is only really an issue if the editors can
find the poem when googling its title or a couple of lines from the poem
As Alex explains here, bar the thread titles,
everything that's posted on the critical fora is made invisible to Google and other search engines, so if you're worried, then use a thread title that isn't your poem title. You'll see that happening a fair bit here.
I recently discovered a slight exception to the above,
detailed here. A nasty site called rssing.com are 'scraping' (i.e. taking) the first 150 characters of threads and reposting
them here where they then do show up in Google. So your poem (at least the first lines of it) could be found this way. If that worries you, prefix the your poem thread with 150 whited out characters (spaces don't count). Alex has asked rssing.com to desist, but they've not done so yet.
All of the above may be superfluous because inactive poem threads are deleted on a monthly basis. So by the time the editor gets around to reading the poem, even if she finds the a reference to your poem on Google, it will lead nowhere. The poem itself will no longer be on the site, which I imagine will be good enough for most editors (I checked this out with Tim Green, Rattle's editor and he said it would be fine for him, despite having an "if it's on the web it's published policy"). You can also ask a moderator to delete your poem thread if the monthly cull doesn't seem soon enough.
Some people believe that the precautions suggested above are mere foolishness and a waste of time and may post below to say so. Others disagree. I say do whatever you need to do to feel relaxed about posting.
I look forward to seeing your poems.
Matt