Here's the fun part: once you have the DropDAV account set up, launch any one of the iWork for iPad apps. While in the My Documents screen, tap the Copy button at the bottom of the page -- it's the one that looks like an arrow pointing to an inbox. Select Copy from WebDAV, and you'll be prompted to enter in a server address, user name, and password for a WebDAV server (see below). In those slots, enter
https://dav.dropdav.com, your Dropbox user name, and your Dropbox password, then press the Sign In button.
If everything was done properly, the next thing you should see is a list of the folders in your Dropbox (see below). You can open the folders by tapping on them and navigate through subsequent folders with taps. When you find the document you want to copy to your iPad, tap on it, and it is copied to the iPad and opened. Pages can open Pages for Mac, text, and Microsoft Word documents, Numbers can open Numbers for Mac and Excel documents, and Keynote can open Keynote for Mac and PowerPoint documents.
Once you've made your edits using iWork for iPad and you're ready to save them back to your Dropbox, just tap the Share icon (the one at left in the gray icons above), then tap the Copy to WebDAV button. If you're not signed into the DropDAV WebDAV server, you'll be prompted to sign in as noted earlier in this post. If you are signed in, you'll be asked to copy the document in one of several formats. For Pages, you'll have a choice of Pages, PDF, or Word .doc files. For Numbers, the choices are Numbers, PDF, or Excel .xls formats, and for Keynote, the choices are Keynote, PDF, or PowerPoint .ppt (see below).
Make the choice of format, then tap the Copy button. If the file already exists in your Dropbox, you'll be asked if you wish to replace it, in which case it will be overwritten with the edited file from your iPad.
Personally, I've been waiting for this capability since iWork for iPad came out, and now it's possible. Chances are very good that Apple will never add native support for Dropbox to iWork for iPad -- after all, they didn't invent Dropbox -- so DropDAV will continue to be a super way to shuttle between Dropbox and your iPad.