Well most BT clients have user-configurable port settings (or just pick a random one) so it's likely that whatever port-forwarding you have set up will not apply to the Blizzard BT client.
Here, grabbed this off their site:
Quote:
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The Blizzard Downloader requires that ports 3724 and 6112 be open. It can also benefit from having ports 6881 through 6999 open.
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Hope that helps. I'm sure that the majority of problems users are having with download speeds stem from an improperly configured firewall.