one or the other, but not both. i hate splitting my work between two computers. (i know - i should invest in some syncing software)
i love my laptop for its mobility, however i do use the desktop much more often when at home. also i bounce back and forth internationally time to time, so a laptop is nice because i have 'my' computer with me when i travel. even for short trips it is not really a hassle, and when i travel for extended periods it really becomes a blessing. i never use it on the plane though, unless the plane is empty because i am considerate of other passengers who are already crammed in economy class.
i would much rather have a HTPC and a laptop though. i am currently in the planning phase of converting my desktop into a HTPC. (need a HDTV first, needless to say going to set back my project even longer)