I've been reading a lot of Subversion books and documentation, while I wait for my hard drive to arrive. I found a major design flaw in SVN Backup Widget's implementation of the incremental dump feature. I totally misunderstood how this worked in the "svnadmin dump" command.
I had implemented it as the incremental file being a separate entity from a full dump file. Subversion uses a dump file as the "root", then uses deltas to include the new changes in this root dump file. That's the reason for the Checksum Mistmatch error that some people have been getting. SVN Backup Widget is creating an individual file, when it should have been appending to an existing dump file.
I need to rethink how to do the UI for this, so that I can cover the correct usage of this feature. So please, do not use the incremental feature until I get it fixed.