Definitely. Entire sets of account data — time, tasks, milestones, projects, etc. — can be exported in XML and CSV formats by navigating to Options >> Settings & Defaults >> Export Data. At this time the only data that cannot be bulk exported is documents. Documents can be individually downloaded.
Customized backups can be built by utilizing the API.
Additionally, most pages and time reports contain links to export the current page to PDF or CSV. The CSV format is useful for manipulating the data in a spreadsheet program or importing into another application.
Data can also be imported. Navigate to Options >> Settings & Defaults >> Import Data to get started. Clients, projects, tasks, people, and milestones can be imported from three different text formats — comma delimited (CSV), tab delimited (TSV) and pipe delimited.