I have a desktop and a laptop and I like to keep the same iTunes music library (~20GB) on both. I use a great freeware program called Sync <http://www.decimus.net/synk/>. Synk is actively developed and the author responds quickly to questions.

One thing that I've found is that it best to have a "master" library, say on the desktop, and only make changes there. Then periodically I just run a one way Synk (a backup, if you will) from the desktop to the laptop. Synk only copies files that have changed, and you can set an option to overwrite files if the mod dates is newer, which forces the xml files in the iTunes music folder to copy.

In the current version of iTunes it is possible to make a playlist of songs that you have dragged across from the iTunes shop. These obviously have hyperlinks to the shop embedded within. There does not seem to be
an obvious way to export this play list (which is different because it
does not link to songs on your physical drive) and easily bring it to
another machine. For example you might have collected a set of songs
you want to consider purchasing and then you want to take that from
home to work and every now and again listen to the preview and
decide whether you will buy.