In "How an Oregon lawyer uses Highrise to manage clients and Backpack to GTD," Joshua M. Barrett discusses how he uses the Satchel app. He uses it to bring Backpack to his iPhone and iPad in order to manage to-do items and pages while on the go.
Here are a couple of reviews of Satchel.
While you may be a little hesitant to drop the $9.99US on this one, I can say that it's worth every penny for the true Backpack fanatic. It's gone a long way to removing the barrier for those looking to embrace Backpack as a service, but feeling a little hamstrung by the lack of a decent mobile interface. If you love Backpack, you'll love Satchel [App Store].
[Satchel] is an excellent tool for anyone seeking to organize a business, club, family, or other type of group where everyone needs to be kept up to date about projects and tasks. It serves as a good replacement for the paper-based calendars or stream of e-mails that some organizations still use to try to maintain contact.
There are other Backpack Extras too. (Note: Satchel and these other Extras are third-party products. They are not made by 37signals.)