tedster - 7:41 pm on Sep 16, 2010 (gmt 0)
If you want both URL versions of the forum to get search traffic, yes - you do have a duplicate URL worry in that only one version will be ranked for any given query. It might even be a crap shoot as to which one, although the original version will probably have the edge.
For Google, you could use a canonical link tag that points to the other domain, so that way you suggest which domain gets into the index. However, at least for now, Google is the only search engine that supports cross-domain canonical tagging.