You too?
I'm having the same problem and its annoying as hell! I can't find a setting to turn off that default app check anywhere in Acrobat Pro 8. Everytime I open a file in Acrobat I have to say no to it. Why? Because I love Preview as the default, opens files in a fraction of the time it takes Adobe.
I'd uninstall the thing if it weren't for the fact I need Acrobat & the Pitstop Pro plugin to edit PDF files. This thread is from May - surely someones found a way to deactivate the default check?