You can create Quartz filters to do this in Automator. A bit complicated, but a Google search may be able to help you.
EDIT:
Here's one I prepared earlier for myself:
http://dl.dropbox.com/u/10569001/Compress PDF to 150dpi.qfilter
Copy the Quartz Filter file to ~/Library/Filters. The next time you save a file as PDF, in the Save As dialog, there should be a Quartz Filter drop down menu, choose "Compress PDF to 150dpi" from the menu (same as the file above) and then save the file. It should come out at a much smaller size, depending on how big the source images are.
OS X actually already has a preset Reduce File Size filter, but it basically puts the compression settings to the extreme, so you end up with a small PDF with basically useless embedded images. Shame, because they could've made a really handy filter that'd save people time from doing it themselves.