What's in the "Other" category on my iPad?
The stuff in the Other category is typically documents that have been deleted on the iPad, and also documents that are in use by various apps (for example, PDF documents that are in use by GoodReader, or docs from the iWork programs). Sometimes these documents are available via the Apps tab in iTunes, other times not; it depends on the program. For example, Documents to Go and GoodReader can manage their own documents outside of iTunes using Dropbox.

Deleted documents should be cleared out when you sync with iTunes. Sometimes, due to a known iTunes bug, the Other category won't shrink as expected.

There's more at these Apple Discussions threads:


http://discussions.apple.com/thread.jspa?messageID=12908823&#12908823
http://discussions.apple.com/thread.jspa?threadID=2715603