Port the data in your Backpack account to a local Gollum wiki.
I hacked this up after I realized that Backpack won't let me download my own uploaded files via their API and I decided to migrate to Gollum+Dropbox (of course, I have to manually download the files...).
git clone git://github.com/swaroopch/backpack_to_gollum.git
cd backpack_to_gollum
bundle install
# Download the export.xml from <your-account>.backpackit.com/account/exports
ruby backpack_to_gollum.rb
cd notes
gollum