1) I'm going to assume you've already upgraded your system to 10.6.5 and that you're running the latest iOS 4.2 GM on your device (or the official version, when it's eventually released).
2) If you haven't already, Hacktivate AirPrint on the system you'll use to serve printers. That is, unless Apple re-activates the feature by the time you read this. Either way, you'll need AirPrint up and running. Reboot.
3) Install CUPS-PDF on the AirPrint server system. This will allow you to create a PDF "printer" on the system. Installation is rather painless.
4) Create the CUPS-PDF printer by using System Preferences->Fax & Print. Click the '+' sign to add a new printer, and you'll see a similar dialog box as pictured to the right (minus my Canon printer). Select CUPS-PDF and name the printer whatever you want, using the Generic PostScript Printer selection. Click 'Add'.
5) With the new "printer" selected in the 'Print & Fax' screen, check the 'Share this printer on the network' box.
5.5) On your iOS device running 4.2, you can now enter an app that supports printing and choose the new PDF printer. Print something to it, to create the directory we'll move later.
6) Install Dropbox if you don't already use it.
7) Using the Terminal, you're going to move aside the original directory that CUPS-PDF uses to store PDFs and have them instead appear in your Dropbox:
$ mkdir ~/Dropbox/Documents/PDFs
$ sudo mv /var/spool/cups-pdf/ANONYMOUS /var/spool/cups-pdf/ANONYMOUS-orig
$ sudo ln -s /var/spool/cups-pdf/ANONYMOUS ~/Dropbox/Documents/PDFs
8) Now when you print to this device, the generated file will appear in your Dropbox (in the folder you specified in Step 6). Open with the Dropbox app, GoodReader, or whatever else you're using that supports Dropbox integration, and voila, a PDF printout of whatever you were looking at, good to go!