The replays are located in your user path (Docs and Settings AppData\Local\ on XP, or User \AppData\Local\Ironclad Games\Sins of a Solar Empire\ on Vista), in the appropriate folder, either AutoRecord-Multiplayer or Singleplayer.
To post a replay, you can go to wincustomize.com (Stardock's file hosting system), use your Stardock (this forum's) login info and submit to gallery > Sins Replays.
Alternatively, if it's a bug replay or something you don't intend as a showcasing replay, you can just use a free file hosting site like rapidshare.com, or simply zip it up and email it to
[email protected] if it's something you want them to look at.
I'm tired at work, but hopefully the rambling makes sense