If there was already an existing and readily available fly database I don't think this would be too hard. But as Jeff says, the biggest part is entering all the fly patterns and materials list for each one. I do kind of miss the point of tying based on the materials I have at hand though.... I don't think I've ever sat down and said what can I tie with this stuff, its usually more of how can I tie this fly with this stuff...

Also, I would say to be completely useful this type of DB would have to have pictures and maybe even step by steps unless it's just being used for ideas and any other references needed would be found elsewhere.