With iTunes 9.2 and iOS 4, the iBooks 1.1 app for iPad, iPhone and iPod touch can store and view PDF files, together with EPUB files.
If you want to quickly add a document or a web page to your 'Books' collection in iTunes, all you need to do is to create an alias of iTunes and drag it to ~/Library/PDF Services. Now, when you're browsing the web or viewing documents and you decide that you want to read them later on a portable Apple device just hit Print, click the PDF button on the bottom left corner of the window and choose iTunes. iTunes will launch and receive the PDF. Next, sync your device and you're ready to go.
[crarko adds: I tested this, and it works as described. This idea will work with any application that can read a PDF, not just iTunes 9.2.]
Mac OS X Hints