not easily
Have you thought about upgrading your laptop's hard drive? If you only have 30GB of movies, that would be your easiest solution especially if you can replace the hard drive yourself through a website like ifixit.
Okay, so there are actually two locations for iTunes libraries. There's the location that you set and the default location. If you set the library on an external drive and it is later no longer available, iTunes will automatically file any uploads to the default iTunes location. Later when connected to the external library, I don't have to do anything other than rebooting iTunes to reconnect to the master library (unsure if that's necessary all the time, but it takes care of the "I can't find this" exclamation point). This allows the possibility of a workaround, but first a warning... there's a key part here that might cause your iTunes library to double which is a HUGE pain to deal with especially if you use playlists extensively.
hook up your external drive.
open iTunes, open Preferences from the iTunes (file on Windows?) menu
select the Advanced Tab, select "Keep iTunes Music folder organized" and "Copy files to iTunes Music folder when adding...", and change the iTunes library to the desired location on the external. Press "Okay". IMPORTANT: When it asks you whether you want it to move and rename the files in your new iTunes Music folder to match the 'Keep iTunes Music folder organized' preference?" say *NO*.
you'll want to do one file as a test, because this part will take a while. Basically, you are going to move all the movies to your external drive. Again in iTunes but this time from the File menu, select "Add to Library" (note that this is also how you import things so that they don't automatically play when you add them). It will bring up a file browser and you'll move to "import" your first movie. This is a better way of doing it than manually moving the files because it will move all the metadata with it and it will keep track of where it thinks the file is. Since the libraries are set up on separate drives, it will just make a copy of your movie. Before deleting the original file on your laptop, I would recommend first checking that the target file is the same size (might have to divide by 1.024 memory conversion error?) and is where you would anticipate it going in your new iTunes music folder. To be super safe, check the info in iTunes (command-i, or File>Get-Info) to make sure that it knows it is located in the new library too.
Now that you've got the hang of it, you can do the same "Add to Library" command, but select all the remaining movies at once rather than one by one. If you have your iTunes set to automatically update podcasts, you might end up with them on your laptop or your external depending on whether it was hooked to the computer when iTunes was opened. You can still import movies into iTunes while on the road and they will go to the default location on the laptop. This is easiest on a day-to-day basis anyway unless you're working at a desk. Moving them to the external drive would be exactly as above.
The same procedure can be used to selectively move files from the external to the laptop with the appropriate adjustments (ie, moving the library location back to the laptop and "importing" the files you want to move, then moving the library back.)
It's really not _that_ bad, but you can see why I would recommend you buy a new hard drive if you can make room for it in your budget! Ideally of course, iTunes would allow you to set different library locations for different media types--or at least movies. Given the sizes of the AppleTV, I wouldn't hold my breath! I feel that they're not serious about ipod video until the hardware will work with 720p anyway. Do my friends really want to watch VHS quality movies on their big screen high definition television?
Good luck,
M@
