All the established Doctor Who news sites are now covering Torchwood news as well. Outpost Gallifrey and UNIT News have both setup special Torchwood sections.
There are a handful of other Torchwood fansites around including The Institute, http://www.sylvestermccoy.com/torchwood/, and Torchwood.org.
The most thriving Torchwood community is perhaps at The Torchwood Institute on Live Journal. There is also some Torchwood talk at Torchwood Online plus chatter at all the usual Doctor Who forums.
My favorite of the batch however, which will no doubt provide months of entertainment until the show premiers is YTMND:Torchwood. Feel free to post any sites I may have missed in the comments below.