The GTD just released its latest an open-source database -- structured (& unstructured) details about 80K terrorist attacks from 1970 through 2007 (with annual updates planned for the future). It includes systematic data on domestic as well as international terrorist incidents that have occurred during this time period. The previous dataset provided info through 2004. You can see the latest one here.
The site is designed for interactive research through the web. But as the FAQ explains: "United States government officials and interested researchers can request a copy of the GTD data files from the March 2009 release through the GTD Contact Form."
There is also a nifty interactive visualization tool called "Data Rivers." Click through below to play with it.