About a month ago, if you want access to your Dropbox files from your Windows Mobile phone, your only option is Ruttensoft's CloudFiles. Now, they have a competitor - Droppedboxx. It's not as fully featured as CloudFiles, but it gets the job done, especially if all you want to do is ftp between your phone and your Dropbox account.
The only feature I miss from CloudFiles is the internal text editor, but I can live without it. Price wise, the official releases for Cloud Files and Droppedboxx are $6.99 and $2.99 respectively.