i use my mbp for
Web Surfing
Newsgroups
Torrent files
As an htpc to stream wmv movies to the xbox 360
watch videos
ichat
listen to music
take it around the house, outside anywhere
I also have a 2.8ghz 8 core mac pro with 16gb of RAM and I use that mainly for downloading encoding hd content, ripping, everything above listed and whatever goes and storage of music and movies.
Then whenever I want to use to watch to stream 1080p/720p movies onto my 1080p hdtv, I load the movie up on the mbp and just stream from the mbp to the xbox 360 and put the mac pro to sleep (since the mp is in the same room where my 52" sony bravia and 2 receivers and power amp and the xbox 360, all that makes the room warm especially with the mac pro running, so I just put the mp to sleep and use the mbp to stream. =D