I hate that Safari will automatically present PDFs for viewing; every time you browse back to the page, you have wait for the document to be downloaded again; it does not persist if you disconnect from the network; etc.
While I had used the Terminal command defaults write com.apple.Safari WebKitOmitPDFSupport -bool YES with success for a long while, it apparently was broken by the recent release of Acrobat 7.0.7.
However, by navigating to /Library/Internet Plug Ins, I simply moved the (updated) AcrobatPDFViewerPlugin to the "Disabled Plug-ins" folder. Relaunched safari, and relaunched Acrobat, and it seems like the choice is respected -- now PDFs download like they should.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060315100902326