The IOM (Institute of Medicine) released a report in October 2001 which suggests that caffeine can be used safely to improve alertness and cognitive performance on some tasks. While this is hardly news to the billions of caffeine users around the world, it is of note because:

• They suggest high dosages: 100-600 mg
• They recommend it should be used by soldiers
• They suggest it is quite safe, include a review of the medical literature

Definitely worth checking out:

Summary of IOM's Report on Caffeine and Mental Performance
Complete Report - (in a horrible format, hopefully they will have a better version soon)