If you're not satisfied with Google's cute but data-sucking incarnation of Google Docs through mobile Safari, you might want to download MiGhtyDocs. This free application on the app store will pull down all of your documents and spreadsheets from the service, making them available to read and access even when away from a data connection.
All you have to do to get any document cached for offline viewing is open it once. If there are any changes since the last time you accessed it, they'll be download the next time it syncs back up with Google's servers. If you're worried about privacy you can also completely clear out the cache whenever you please.
Despite its charms, the application is missing a few things that would make it the holy grail of Google Docs tools. It's lacking support for Google's latest Docs addition--presentations. It also always remembers your account information, so if your phone gets stolen someone could get extensive access to your work. That said, you can't create new items, rename files, or send anything to someone else through e-mail, so you don't have to worry about people changing or distributing existing work.
I still like that you can grab something large like an expense sheet or eBook from Google Docs, then access it while offline. It makes for a handy utility if you're willing to approach it as a middleman for your content. Developer Thomas Post (aka v1ru8) is also the creator or two other applications, iPhoneNotes and the upcoming Marks.