I think I've found a good combination for streaming to the Tab. I used to have an Archos 70IT, which had native smb and upnp support, which worked great with my upnp server. The software I am using is PS3 Media Server (java-based, so multiplatform capable: http://code.google.com/p/ps3mediaserver/
). At first, I tried using AllShare, which sucks as a renderer. I couldn't make it work with PMS, regardless of settings in the renderer config file snippet I had grabbed from their forums.
Not to be completely beaten, I took a recommendation from this thread to install UpnPlay from the Market, as well as VPlayer. UpnPlay has its own user-agent string, which is not entirely needed in PMS, as you use it only to browse the media. To actually stream something, it sends a different user-agent string, which I've captured and put into a renderer config file (below). PMS is pretty much set as default as far as transcoding options, and it seems to be able to stream pretty much anything to VPlayer. I also tried the most recent RockPlayer, and while it has a nicer interface, the performance is sub-par. You can't enable the hardware decoding in RP unless your transcoded files are of a format that the stock video player can decode.
# ps3mediaserver renderer profile for Samsung Galaxy Tab
# Refer to PS3.conf for help
RendererName=Samsung Galaxy Tab Vplayer
Supported=f:avi|divx v:mp4|divx|mjpeg a:mp3|lpcm|mpa|ac3 m:video/x-msvideo qpel:no gmc:0
EDIT: This same setup works great on my Captivate too