As the PDF plugin is currently PowerPC only, it does not work on Intel Macs. The only workaround is to run the browser within Rosetta. For Safari and Firefox, this is not a huge problem as both are able to view inline PDFs (the first with its own engine, and both via Adobe Reader 8). Sadly, this doesn't work with Camino which is quite annoying.
As QuickTime is able view PDFs, I tried to change the MIME type settings in order to let the QuickTime plug-in handle PDF files. Sadly, the necessary setting in the QuickTime Preference Pane doesn't seem to stick, or Camino seems to ignore it.
I finally discovered (through the help of the Camino Mailing List) that Camino also stores this information in the file pluginreg.dat (to be found at ~/Library -> Application Support -> Camino). So if you add an entry which forwards PDFs to the QuickTime plug-in, you're finally able to view PDFs inline again.
The line has to be added to the end of the QuickTime Plug-in section, and looks like this:
nn:application/pdf
DF image
df:$
Where nn is the next free number within this section. In my case, that was 62. You also have to increase the counter at the beginning of the section by one (in my case to 63, as the list starts with 0).
Sadly nothing is without disadvantages: You will only see the first page of the PDF, as the QuickTime controller is not shown, and zooming is not possible. Hopefully Adobe will include Camino support in future releases of Adobe Reader, or the PDF Browser Plug-in will be released as a Universal Binary.