The United States Institute of Peace, which I admit I'd never prviously heard of, have issued a paper saying terrorists use the Internet too but for more routine activities than the hyped-up cyberterrorism feedstuff of the mainstream media. Terrorist organisations are said to have three major audiences:
Current and potential supporters
International public opinion
Enemy publics (i.e. citizens of states they are fighting)
and use the Net in 8 different (sometimes overlapping) ways:
Psychological warfare
Publicity and propaganda
Data mining
Fundraising
Recruitment and mobilisation
Networking
Sharing information
Planning and coordination

The report does imply that steganography is in widespread use by terrorist organisations but there is no direct evidence offered to that effect. The mainstream media has periodically salivated at the notion of religeous fundamentalist terrorists hiding messages in online porn but no evidence to that effect has been forthcoming. Being only 12 pages long the paper is just an overview I assume but it might be interesting to hear more details of the study.