How can I convert a ppt to a pdf from the command line?

You can also use the command line of libreoffice for your purpose. This example converts all ppt-files in the current directory to pdf's:

libreoffice --headless --invisible --convert-to pdf *.ppt  

--headless
Starts in "headless mode", which allows using the application without a user interface.
This special mode can be used when the application is controlled by external clients via the API...
It implies --invisible and strictly ignores any GUI environment.

and

--invisible
Starts in invisible mode.
Neither the start-up logo nor the initial program window will be visible. LO can be controlled and documents and dialogs can be opened via the API.
When started with this parameter, it can only be quit using the taskmanager (Windows) or the kill command (UNIX based systems).

Get more information on command line options with:

man libreoffice

(Note: You have to close all running instances of LibreOffice before the command line works.)

You can use unoconv. It's available in the repositories, so you can just apt-get it: unoconv (direct install link)

Note that the info in the web page is outdated. Use man or type unoconv -h to get actual command line options. It works like this:

unoconv -f pdf presentation.odt

This command will create a presentation.pdf file.

I have only used it to batch convert ods to pdf and it works perfectly. I haven't tried it with presentations (but for a quick test now), but I expect that it will work nicely.

Note that it will not work on Lucid (10.04), because of a bug in the python-openoffice biddings. If you need to use it on Lucid, you can add this ppa to your software sources to upgrade your Openoffice version.

The easiest way to convert .ppt files to PDF is to install unoconv through the command line:

sudo apt-get install unoconv

Then open the terminal from the drive where the ppt files are available. Now type:

unoconv -f [format] pdf  filename.ppt

That's all. Done.