Just to add a little to that.... I use my SD card in my BB for screensaver images and web browser bookmarks (.urls) which are scanned as photos. I transfer everything to it from a PC, it was plugged in the first time I turned the BB on and it's never been removed. Once you turn on the sharing in the BB it just shows up in windows explorer on the network, just double click, enter username/password and use it like any other attached storage (as does any BB attached USB drive). It's slow but for what I transfer it doesn't matter (although I do use wireless but I still hear its slow).
No, the current Boxee Box software does not use the SD card for anything. If you are thinking of building apps you can put test ones on it I believe... or it may only be for the USB attached drive. http://forums.boxee.tv/showthread.php?t=28972
As far as I am aware there is no BB torrent client app.
Running Boxee Box firmware: 188.8.131.5235